06.02.2019 15:48, Thierry Reding пишет:
> On Wed, Feb 06, 2019 at 03:40:52PM +0300, Dmitry Osipenko wrote:
>> 06.02.2019 1:46, Sowjanya Komatineni пишет:
>>> This patch adds DMA support for Tegra I2C.
>>>
>>> Tegra I2C TX and RX FIFO depth is 8 words. PIO mode is used for
>>> transfer size of the max FIFO depth and DMA mode is used for
>>> transfer size higher than max FIFO depth to save CPU overhead.
>>>
>>> PIO mode needs full intervention of CPU to fill or empty FIFO's
>>> and also need to service multiple data requests interrupt for the
>>> same transaction. This adds delay between data bytes of the same
>>> transfer when CPU is fully loaded and some slave devices has
>>> internal timeout for no bus activity and stops transaction to
>>> avoid bus hang. DMA mode is helpful in such cases.
>>>
>>> DMA mode is also helpful for Large transfers during downloading or
>>> uploading FW over I2C to some external devices.
>>>
>>> Signed-off-by: Sowjanya Komatineni <skomatineni@xxxxxxxxxx>
>>> ---
>>> [V12] : Replaced dma_alloc_coherent with dma_alloc_attrs to force the allocated
>>> buffer to be contiguous also in physical memory as Tegra194 supports max
>>> 64K and dma_alloc_coherent doesnt guarentee contiguous memory.
>>> Changed return code from EIO to EINVAL incase of failure to obtain dma
>>> descriptor.
>>> Fixed coding style check issues.
>>> [V11] : Replaced deprecated dmaengine_terminate_all with dmaengine_termine_async
>>> from non-atomic context and dmaengine_terminate_sync from atomic context.
>>> Fixed to program fifo trigger levels properly when transfer falls back to
>>> pio mode in case of dma slave configuration failure and other minor fixes.
>>> [V10] : APBDMA is replaced with GPCDMA on Tegra186 and Tegra194 designs.
>>> Added apbdma hw support flag to now allow Tegra186 and later use
>>> APBDMA driver.
>>> Added explicit flow control enable for DMA slave config and error handling.
>>> Moved releasing DMA resources to seperate function to reuse in
>>> multiple places.
>>> Updated to register tegra_i2c_driver from module level rather than subsys
>>> level.
>>> Other minor feedback
>>> [V9] : Rebased to 5.0-rc4
>>> Removed dependency of APB DMA in Kconfig and added conditional check
>>> in I2C driver to decide on using DMA mode.
>>> Changed back the allocation of dma buffer during i2c probe.
>>> Fixed FIFO triggers depending on DMA Vs PIO.
>>> [V8] : Moved back dma init to i2c probe, removed ALL_PACKETS_XFER_COMPLETE
>>> interrupt and using PACKETS_XFER_COMPLETE interrupt only and some
>>> other fixes
>>> Updated Kconfig for APB_DMA dependency
>>> [V7] : Same as V6
>>> [V6] : Updated for proper buffer allocation/freeing, channel release.
>>> Updated to use exact xfer size for syncing dma buffer.
>>> [V5] : Same as V4
>>> [V4] : Updated to allocate DMA buffer only when DMA mode.
>>> Updated to fall back to PIO mode when DMA channel request or
>>> buffer allocation fails.
>>> [V3] : Updated without additional buffer allocation.
>>> [V2] : Updated based on V1 review feedback along with code cleanup for
>>> proper implementation of DMA.
>>>
>>>
>>> drivers/i2c/busses/i2c-tegra.c | 413 ++++++++++++++++++++++++++++++++++++-----
>>> 1 file changed, 369 insertions(+), 44 deletions(-)
>>>
>>> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
>>> index 118b7023a0f4..77277a09e485 100644
>>> --- a/drivers/i2c/busses/i2c-tegra.c
>>> +++ b/drivers/i2c/busses/i2c-tegra.c
>>> @@ -8,6 +8,9 @@
>>>
>>> #include <linux/clk.h>
>>> #include <linux/delay.h>
>>> +#include <linux/dmaengine.h>
>>> +#include <linux/dmapool.h>
>>> +#include <linux/dma-mapping.h>
>>> #include <linux/err.h>
>>> #include <linux/i2c.h>
>>> #include <linux/init.h>
>>> @@ -44,6 +47,8 @@
>>> #define I2C_FIFO_CONTROL_RX_FLUSH BIT(0)
>>> #define I2C_FIFO_CONTROL_TX_TRIG_SHIFT 5
>>> #define I2C_FIFO_CONTROL_RX_TRIG_SHIFT 2
>>> +#define I2C_FIFO_CONTROL_TX_TRIG(x) (((x) - 1) << 5)
>>> +#define I2C_FIFO_CONTROL_RX_TRIG(x) (((x) - 1) << 2)
>>> #define I2C_FIFO_STATUS 0x060
>>> #define I2C_FIFO_STATUS_TX_MASK 0xF0
>>> #define I2C_FIFO_STATUS_TX_SHIFT 4
>>> @@ -125,6 +130,19 @@
>>> #define I2C_MST_FIFO_STATUS_TX_MASK 0xff0000
>>> #define I2C_MST_FIFO_STATUS_TX_SHIFT 16
>>>
>>> +/* Packet header size in bytes */
>>> +#define I2C_PACKET_HEADER_SIZE 12
>>> +
>>> +#define DATA_DMA_DIR_TX BIT(0)
>>> +#define DATA_DMA_DIR_RX BIT(1)
>>> +
>>> +/*
>>> + * Upto I2C_PIO_MODE_MAX_LEN bytes, controller will use PIO mode,
>>> + * above this, controller will use DMA to fill FIFO.
>>> + * MAX PIO len is 20 bytes excluding packet header.
>>> + */
>>> +#define I2C_PIO_MODE_MAX_LEN 32
>>> +
>>> /*
>>> * msg_end_type: The bus control which need to be send at end of transfer.
>>> * @MSG_END_STOP: Send stop pulse at end of transfer.
>>> @@ -166,6 +184,7 @@ enum msg_end_type {
>>> * allowing 0 length transfers.
>>> * @supports_bus_clear: Bus Clear support to recover from bus hang during
>>> * SDA stuck low from device for some unknown reasons.
>>> + * @has_apb_dma: Support of APBDMA on corresponding Tegra chip.
>>> */
>>> struct tegra_i2c_hw_feature {
>>> bool has_continue_xfer_support;
>>> @@ -180,6 +199,7 @@ struct tegra_i2c_hw_feature {
>>> bool has_mst_fifo;
>>> const struct i2c_adapter_quirks *quirks;
>>> bool supports_bus_clear;
>>> + bool has_apb_dma;
>>> };
>>>
>>> /**
>>> @@ -191,6 +211,7 @@ struct tegra_i2c_hw_feature {
>>> * @fast_clk: clock reference for fast clock of I2C controller
>>> * @rst: reset control for the I2C controller
>>> * @base: ioremapped registers cookie
>>> + * @base_phys: Physical base address of the I2C controller
>>> * @cont_id: I2C controller ID, used for packet header
>>> * @irq: IRQ number of transfer complete interrupt
>>> * @irq_disabled: used to track whether or not the interrupt is enabled
>>> @@ -204,6 +225,13 @@ struct tegra_i2c_hw_feature {
>>> * @clk_divisor_non_hs_mode: clock divider for non-high-speed modes
>>> * @is_multimaster_mode: track if I2C controller is in multi-master mode
>>> * @xfer_lock: lock to serialize transfer submission and processing
>>> + * @tx_dma_chan: DMA transmit channel
>>> + * @rx_dma_chan: DMA receive channel
>>> + * @dma_phys: handle to DMA resources
>>> + * @dma_buf: pointer to allocated DMA buffer
>>> + * @dma_buf_size: DMA buffer size
>>> + * @is_curr_dma_xfer: indicates active DMA transfer
>>> + * @dma_complete: DMA completion notifier
>>> */
>>> struct tegra_i2c_dev {
>>> struct device *dev;
>>> @@ -213,6 +241,7 @@ struct tegra_i2c_dev {
>>> struct clk *fast_clk;
>>> struct reset_control *rst;
>>> void __iomem *base;
>>> + phys_addr_t base_phys;
>>> int cont_id;
>>> int irq;
>>> bool irq_disabled;
>>> @@ -226,6 +255,13 @@ struct tegra_i2c_dev {
>>> u16 clk_divisor_non_hs_mode;
>>> bool is_multimaster_mode;
>>> spinlock_t xfer_lock;
>>> + struct dma_chan *tx_dma_chan;
>>> + struct dma_chan *rx_dma_chan;
>>> + dma_addr_t dma_phys;
>>> + u32 *dma_buf;
>>> + unsigned int dma_buf_size;
>>> + bool is_curr_dma_xfer;
>>> + struct completion dma_complete;
>>> };
>>>
>>> static void dvc_writel(struct tegra_i2c_dev *i2c_dev, u32 val,
>>> @@ -294,6 +330,111 @@ static void tegra_i2c_unmask_irq(struct tegra_i2c_dev *i2c_dev, u32 mask)
>>> i2c_writel(i2c_dev, int_mask, I2C_INT_MASK);
>>> }
>>>
>>> +static void tegra_i2c_dma_complete(void *args)
>>> +{
>>> + struct tegra_i2c_dev *i2c_dev = args;
>>> +
>>> + complete(&i2c_dev->dma_complete);
>>> +}
>>> +
>>> +static int tegra_i2c_dma_submit(struct tegra_i2c_dev *i2c_dev, size_t len)
>>> +{
>>> + struct dma_async_tx_descriptor *dma_desc;
>>> + enum dma_transfer_direction dir;
>>> + struct dma_chan *chan;
>>> +
>>> + dev_dbg(i2c_dev->dev, "starting DMA for length: %zu\n", len);
>>> + reinit_completion(&i2c_dev->dma_complete);
>>> + dir = i2c_dev->msg_read ? DMA_DEV_TO_MEM : DMA_MEM_TO_DEV;
>>> + chan = i2c_dev->msg_read ? i2c_dev->rx_dma_chan : i2c_dev->tx_dma_chan;
>>> + dma_desc = dmaengine_prep_slave_single(chan, i2c_dev->dma_phys,
>>> + len, dir, DMA_PREP_INTERRUPT |
>>> + DMA_CTRL_ACK);
>>> + if (!dma_desc) {
>>> + dev_err(i2c_dev->dev, "failed to get DMA descriptor\n");
>>> + return -EINVAL;
>>> + }
>>> +
>>> + dma_desc->callback = tegra_i2c_dma_complete;
>>> + dma_desc->callback_param = i2c_dev;
>>> + dmaengine_submit(dma_desc);
>>> + dma_async_issue_pending(chan);
>>> + return 0;
>>> +}
>>> +
>>> +static void tegra_i2c_release_dma(struct tegra_i2c_dev *i2c_dev)
>>> +{
>>> + if (i2c_dev->dma_buf) {
>>> + dma_free_attrs(i2c_dev->dev, i2c_dev->dma_buf_size,
>>> + i2c_dev->dma_buf, i2c_dev->dma_phys,
>>> + DMA_ATTR_FORCE_CONTIGUOUS);
>>> + i2c_dev->dma_buf = NULL;
>>> + }
>>> +
>>> + if (i2c_dev->tx_dma_chan) {
>>> + dma_release_channel(i2c_dev->tx_dma_chan);
>>> + i2c_dev->tx_dma_chan = NULL;
>>> + }
>>> +
>>> + if (i2c_dev->rx_dma_chan) {
>>> + dma_release_channel(i2c_dev->rx_dma_chan);
>>> + i2c_dev->rx_dma_chan = NULL;
>>> + }
>>> +}
>>> +
>>> +static int tegra_i2c_init_dma(struct tegra_i2c_dev *i2c_dev)
>>> +{
>>> + struct dma_chan *chan;
>>> + u32 *dma_buf;
>>> + dma_addr_t dma_phys;
>>> + int err;
>>> +
>>> + if (!IS_ENABLED(CONFIG_TEGRA20_APB_DMA) ||
>>> + !i2c_dev->hw->has_apb_dma) {
>>> + err = -ENODEV;
>>> + goto err_out;
>>> + }
>>> +
>>> + chan = dma_request_slave_channel_reason(i2c_dev->dev, "rx");
>>> + if (IS_ERR(chan)) {
>>> + err = PTR_ERR(chan);
>>> + goto err_out;
>>> + }
>>> +
>>> + i2c_dev->rx_dma_chan = chan;
>>> +
>>> + chan = dma_request_slave_channel_reason(i2c_dev->dev, "tx");
>>> + if (IS_ERR(chan)) {
>>> + err = PTR_ERR(chan);
>>> + goto err_out;
>>> + }
>>> +
>>> + i2c_dev->tx_dma_chan = chan;
>>> +
>>> + dma_buf = dma_alloc_attrs(i2c_dev->dev, i2c_dev->dma_buf_size,
>>> + &dma_phys, GFP_KERNEL | __GFP_NOWARN,
>>> + DMA_ATTR_FORCE_CONTIGUOUS);
>>> + if (!dma_buf) {
>>> + dev_err(i2c_dev->dev, "failed to allocate the DMA buffer\n");
>>> + err = -ENOMEM;
>>> + goto err_out;
>>> + }
>>> +
>>> + i2c_dev->dma_buf = dma_buf;
>>> + i2c_dev->dma_phys = dma_phys;
>>> + return 0;
>>> +
>>> +err_out:
>>> + tegra_i2c_release_dma(i2c_dev);
>>> + if (err != -EPROBE_DEFER) {
>>> + dev_err(i2c_dev->dev, "can't use DMA, err: %d, using PIO\n",
>>> + err);
>>> + return 0;
>>> + }
>>> +
>>> + return err;
>>> +}
>>> +
>>> static int tegra_i2c_flush_fifos(struct tegra_i2c_dev *i2c_dev)
>>> {
>>> unsigned long timeout = jiffies + HZ;
>>> @@ -571,16 +712,6 @@ static int tegra_i2c_init(struct tegra_i2c_dev *i2c_dev)
>>> i2c_writel(i2c_dev, 0x00, I2C_SL_ADDR2);
>>> }
>>>
>>> - if (i2c_dev->hw->has_mst_fifo) {
>>> - val = I2C_MST_FIFO_CONTROL_TX_TRIG(8) |
>>> - I2C_MST_FIFO_CONTROL_RX_TRIG(1);
>>> - i2c_writel(i2c_dev, val, I2C_MST_FIFO_CONTROL);
>>> - } else {
>>> - val = 7 << I2C_FIFO_CONTROL_TX_TRIG_SHIFT |
>>> - 0 << I2C_FIFO_CONTROL_RX_TRIG_SHIFT;
>>> - i2c_writel(i2c_dev, val, I2C_FIFO_CONTROL);
>>> - }
>>> -
>>> err = tegra_i2c_flush_fifos(i2c_dev);
>>> if (err)
>>> goto err;
>>> @@ -660,25 +791,37 @@ static irqreturn_t tegra_i2c_isr(int irq, void *dev_id)
>>> if (i2c_dev->hw->supports_bus_clear && (status & I2C_INT_BUS_CLR_DONE))
>>> goto err;
>>>
>>> - if (i2c_dev->msg_read && (status & I2C_INT_RX_FIFO_DATA_REQ)) {
>>> - if (i2c_dev->msg_buf_remaining)
>>> - tegra_i2c_empty_rx_fifo(i2c_dev);
>>> - else
>>> - BUG();
>>> - }
>>> + if (!i2c_dev->is_curr_dma_xfer) {
>>> + if (i2c_dev->msg_read && (status & I2C_INT_RX_FIFO_DATA_REQ)) {
>>> + if (i2c_dev->msg_buf_remaining)
>>> + tegra_i2c_empty_rx_fifo(i2c_dev);
>>> + else
>>> + BUG();
>>> + }
>>>
>>> - if (!i2c_dev->msg_read && (status & I2C_INT_TX_FIFO_DATA_REQ)) {
>>> - if (i2c_dev->msg_buf_remaining)
>>> - tegra_i2c_fill_tx_fifo(i2c_dev);
>>> - else
>>> - tegra_i2c_mask_irq(i2c_dev, I2C_INT_TX_FIFO_DATA_REQ);
>>> + if (!i2c_dev->msg_read && (status & I2C_INT_TX_FIFO_DATA_REQ)) {
>>> + if (i2c_dev->msg_buf_remaining)
>>> + tegra_i2c_fill_tx_fifo(i2c_dev);
>>> + else
>>> + tegra_i2c_mask_irq(i2c_dev,
>>> + I2C_INT_TX_FIFO_DATA_REQ);
>>> + }
>>> }
>>>
>>> i2c_writel(i2c_dev, status, I2C_INT_STATUS);
>>> if (i2c_dev->is_dvc)
>>> dvc_writel(i2c_dev, DVC_STATUS_I2C_DONE_INTR, DVC_STATUS);
>>>
>>> + /*
>>> + * During message read XFER_COMPLETE interrupt is triggered prior to
>>> + * DMA completion and during message write XFER_COMPLETE interrupt is
>>> + * triggered after DMA completion.
>>> + * PACKETS_XFER_COMPLETE indicates completion of all bytes of transfer.
>>> + * so forcing msg_buf_remaining to 0 in DMA mode.
>>> + */
>>> if (status & I2C_INT_PACKET_XFER_COMPLETE) {
>>> + if (i2c_dev->is_curr_dma_xfer)
>>> + i2c_dev->msg_buf_remaining = 0;
>>> BUG_ON(i2c_dev->msg_buf_remaining);
>>> complete(&i2c_dev->msg_complete);
>>> }
>>> @@ -694,12 +837,89 @@ static irqreturn_t tegra_i2c_isr(int irq, void *dev_id)
>>> if (i2c_dev->is_dvc)
>>> dvc_writel(i2c_dev, DVC_STATUS_I2C_DONE_INTR, DVC_STATUS);
>>>
>>> + if (i2c_dev->is_curr_dma_xfer) {
>>> + if (i2c_dev->msg_read)
>>> + dmaengine_terminate_async(i2c_dev->rx_dma_chan);
>>> + else
>>> + dmaengine_terminate_async(i2c_dev->tx_dma_chan);
>>> +
>>> + complete(&i2c_dev->dma_complete);
>>> + }
>>> +
>>> complete(&i2c_dev->msg_complete);
>>> done:
>>> spin_unlock(&i2c_dev->xfer_lock);
>>> return IRQ_HANDLED;
>>> }
>>>
>>> +static void tegra_i2c_config_fifo_trig(struct tegra_i2c_dev *i2c_dev,
>>> + size_t len)
>>> +{
>>> + u32 val, reg;
>>> + u8 dma_burst = 0;
>>> + struct dma_slave_config slv_config = {0};
>>> + struct dma_chan *chan;
>>> + int ret;
>>> +
>>> + if (i2c_dev->hw->has_mst_fifo)
>>> + reg = I2C_MST_FIFO_CONTROL;
>>> + else
>>> + reg = I2C_FIFO_CONTROL;
>>> + val = i2c_readl(i2c_dev, reg);
>>> +
>>> + if (i2c_dev->is_curr_dma_xfer) {
>>> + if (len & 0xF)
>>> + dma_burst = 1;
>>> + else if (len & 0x10)
>>> + dma_burst = 4;
>>> + else
>>> + dma_burst = 8;
>>> +
>>> + if (i2c_dev->msg_read) {
>>> + chan = i2c_dev->rx_dma_chan;
>>> + slv_config.src_addr = i2c_dev->base_phys + I2C_RX_FIFO;
>>> + slv_config.src_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
>>> + slv_config.src_maxburst = dma_burst;
>>> +
>>> + if (i2c_dev->hw->has_mst_fifo)
>>> + val |= I2C_MST_FIFO_CONTROL_RX_TRIG(dma_burst);
>>> + else
>>> + val |= I2C_FIFO_CONTROL_RX_TRIG(dma_burst);
>>> + } else {
>>> + chan = i2c_dev->tx_dma_chan;
>>> + slv_config.dst_addr = i2c_dev->base_phys + I2C_TX_FIFO;
>>> + slv_config.dst_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
>>> + slv_config.dst_maxburst = dma_burst;
>>> +
>>> + if (i2c_dev->hw->has_mst_fifo)
>>> + val |= I2C_MST_FIFO_CONTROL_TX_TRIG(dma_burst);
>>> + else
>>> + val |= I2C_FIFO_CONTROL_TX_TRIG(dma_burst);
>>> + }
>>> +
>>> + slv_config.device_fc = true;
>>> + ret = dmaengine_slave_config(chan, &slv_config);
>>> + if (ret < 0) {
>>> + dev_err(i2c_dev->dev, "DMA slave config failed: %d\n",
>>> + ret);
>>> + dev_err(i2c_dev->dev, "fallbacking to PIO\n");
>>> + tegra_i2c_release_dma(i2c_dev);
>>> + i2c_dev->is_curr_dma_xfer = false;
>>> + } else {
>>> + goto out;
>>> + }
>>> + }
>>> +
>>> + if (i2c_dev->hw->has_mst_fifo)
>>> + val = I2C_MST_FIFO_CONTROL_TX_TRIG(8) |
>>> + I2C_MST_FIFO_CONTROL_RX_TRIG(1);
>>> + else
>>> + val = I2C_FIFO_CONTROL_TX_TRIG(8) |
>>> + I2C_FIFO_CONTROL_RX_TRIG(1);
>>> +out:
>>> + i2c_writel(i2c_dev, val, reg);
>>> +}
>>> +
>>> static int tegra_i2c_issue_bus_clear(struct tegra_i2c_dev *i2c_dev)
>>> {
>>> int err;
>>> @@ -744,6 +964,10 @@ static int tegra_i2c_xfer_msg(struct tegra_i2c_dev *i2c_dev,
>>> u32 int_mask;
>>> unsigned long time_left;
>>> unsigned long flags;
>>> + size_t xfer_size;
>>> + u32 *buffer = NULL;
>>> + int err = 0;
>>> + bool dma;
>>>
>>> tegra_i2c_flush_fifos(i2c_dev);
>>>
>>> @@ -753,19 +977,57 @@ static int tegra_i2c_xfer_msg(struct tegra_i2c_dev *i2c_dev,
>>> i2c_dev->msg_read = (msg->flags & I2C_M_RD);
>>> reinit_completion(&i2c_dev->msg_complete);
>>>
>>> + if (i2c_dev->msg_read)
>>> + xfer_size = msg->len;
>>> + else
>>> + xfer_size = msg->len + I2C_PACKET_HEADER_SIZE;
>>> +
>>> + xfer_size = ALIGN(xfer_size, BYTES_PER_FIFO_WORD);
>>> + i2c_dev->is_curr_dma_xfer = (xfer_size > I2C_PIO_MODE_MAX_LEN) &&
>>> + i2c_dev->dma_buf;
>>> + tegra_i2c_config_fifo_trig(i2c_dev, xfer_size);
>>> + dma = i2c_dev->is_curr_dma_xfer;
>>> +
>>> spin_lock_irqsave(&i2c_dev->xfer_lock, flags);
>>>
>>> int_mask = I2C_INT_NO_ACK | I2C_INT_ARBITRATION_LOST;
>>> tegra_i2c_unmask_irq(i2c_dev, int_mask);
>>> + if (dma) {
>>> + if (i2c_dev->msg_read) {
>>> + dma_sync_single_for_device(i2c_dev->dev,
>>> + i2c_dev->dma_phys,
>>> + xfer_size,
>>> + DMA_FROM_DEVICE);
>>> + err = tegra_i2c_dma_submit(i2c_dev, xfer_size);
>>> + if (err < 0) {
>>> + dev_err(i2c_dev->dev,
>>> + "starting RX DMA failed, err %d\n",
>>> + err);
>>> + goto unlock;
>>> + }
>>> + } else {
>>> + dma_sync_single_for_cpu(i2c_dev->dev,
>>> + i2c_dev->dma_phys,
>>> + xfer_size,
>>> + DMA_TO_DEVICE);
>>> + buffer = i2c_dev->dma_buf;
>>> + }
>>> + }
>>>
>>> packet_header = (0 << PACKET_HEADER0_HEADER_SIZE_SHIFT) |
>>> PACKET_HEADER0_PROTOCOL_I2C |
>>> (i2c_dev->cont_id << PACKET_HEADER0_CONT_ID_SHIFT) |
>>> (1 << PACKET_HEADER0_PACKET_ID_SHIFT);
>>> - i2c_writel(i2c_dev, packet_header, I2C_TX_FIFO);
>>> + if (dma && !i2c_dev->msg_read)
>>> + *buffer++ = packet_header;
>>> + else
>>> + i2c_writel(i2c_dev, packet_header, I2C_TX_FIFO);
>>>
>>> packet_header = msg->len - 1;
>>> - i2c_writel(i2c_dev, packet_header, I2C_TX_FIFO);
>>> + if (dma && !i2c_dev->msg_read)
>>> + *buffer++ = packet_header;
>>> + else
>>> + i2c_writel(i2c_dev, packet_header, I2C_TX_FIFO);
>>
>> I'm now also wondering whether that putting packet_header into the
>> dma_buf is really needed at all.. Isn't possible to push packet_header
>> using PIO regardless of whether the rest of the transfer will be in
>> DMA or PIO modes?
>>
>> Actually I tried to always push packet_header using PIO and apparently
>> DMA works just fine with that:
>
> I don't see a reason why that wouldn't work. After all the DMA support
> here is really just about some external DMA engine writing to the FIFO
> register, so from an I2C controller's point of view it's exactly the
> same thing.
>
> But if we already set up DMA for a transfer, might as well transfer the
> 3 packet header words with that.
Alright!
