On 22/01/18 23:11, Linus Walleij wrote:
> The bounce buffer is gone from the MMC core, and now we found out
> that there are some (crippled) i.MX boards out there that have broken
> ADMA (cannot do scatter-gather), and also broken PIO so they must
> use SDMA. Closer examination shows a less significant slowdown
> also on SDMA-only capable Laptop hosts.
>
> SDMA sets down the number of segments to one, so that each segment
> gets turned into a singular request that ping-pongs to the block
> layer before the next request/segment is issued.
>
> Apparently it happens a lot that the block layer send requests
> that include a lot of physically discontigous segments. My guess
discontigous -> discontiguous
> is that this phenomenon is coming from the file system.
>
> These devices that cannot handle scatterlists in hardware can see
> major benefits from a DMA-contigous bounce buffer.
contigous -> contiguous
>
> This patch accumulates those fragmented scatterlists in a physically
> contigous bounce buffer so that we can issue bigger DMA data chunks
contigous -> contiguous
> to/from the card.
>
> When tested with thise PCI-integrated host (1217:8221) that
thise -> a
> only supports SDMA:
> 0b:00.0 SD Host controller: O2 Micro, Inc. OZ600FJ0/OZ900FJ0/OZ600FJS
> SD/MMC Card Reader Controller (rev 05)
> This patch gave ~1Mbyte/s improved throughput on large reads and
> writes when testing using iozone than without the patch.
>
> dmesg:
> sdhci-pci 0000:0b:00.0: SDHCI controller found [1217:8221] (rev 5)
> mmc0 bounce up to 128 segments into one, max segment size 65536 bytes
> mmc0: SDHCI controller on PCI [0000:0b:00.0] using DMA
>
> On the i.MX SDHCI controllers on the crippled i.MX 25 and i.MX 35
> the patch restores the performance to what it was before we removed
> the bounce buffers, and then some: performance is better than ever
> because we now allocate a bounce buffer the size of the maximum
> single request the SDMA engine can handle. On the PCI laptop this
> is 256K, whereas with the old bounce buffer code it was 64K max.
The current patch requests 64K not 256K.
>
> Cc: Pierre Ossman <pierre@xxxxxxxxx>
> Cc: Benoît Thébaudeau <benoit@xxxxxxxxxxx>
> Cc: Fabio Estevam <fabio.estevam@xxxxxxx>
> Cc: Benjamin Beckmeyer <beckmeyer.b@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v4.14+
> Fixes: de3ee99b097d ("mmc: Delete bounce buffer handling")
> Tested-by: Benjamin Beckmeyer <beckmeyer.b@xxxxxxxxx>
> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
A couple of very minor comments, nevertheless:
Acked-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> ---
> ChangeLog v6->v7:
> - Fix the directions on dma_sync_for[device|cpu]() so the
> ownership of the buffer gets swapped properly and in the right
> direction for every transfer. Didn't see this because x86 PCI is
> DMA coherent...
> - Tested and greelighted on i.MX 25.
> - Also tested on the PCI version.
> ChangeLog v5->v6:
> - Again switch back to explicit sync of buffers. I want to get this
> solution to work because it gives more control and it's more
> elegant.
> - Update host->max_req_size as noted by Adrian, hopefully this
> fixes the i.MX. I was just lucky on my Intel laptop I guess:
> the block stack never requested anything bigger than 64KB and
> that was why it worked even if max_req_size was bigger than
> what would fit in the bounce buffer.
> - Copy the number of bytes in the mmc_data instead of the number
> of bytes in the bounce buffer. For RX this is blksize * blocks
> and for TX this is bytes_xfered.
> - Break out a sdhci_sdma_address() for getting the DMA address
> for either the raw sglist or the bounce buffer depending on
> configuration.
> - Add some explicit bounds check for the data so that we do not
> attempt to copy more than the bounce buffer size even if the
> block layer is erroneously configured.
> - Move allocation of bounce buffer out to its own function.
> - Use pr_[info|err] throughout so all debug prints from the
> driver come out in the same manner and style.
> - Use unsigned int for the bounce buffer size.
> - Re-tested with iozone: we still get the same nice performance
> improvements.
> - Request a text on i.MX (hi Benjamin)
> ChangeLog v4->v5:
> - Go back to dma_alloc_coherent() as this apparently works better.
> - Keep the other changes, cap for 64KB, fall back to single segments.
> - Requesting a test of this on i.MX. (Sorry Benjamin.)
> ChangeLog v3->v4:
> - Cap the bounce buffer to 64KB instead of the biggest segment
> as we experience diminishing returns with buffers > 64KB.
> - Instead of using dma_alloc_coherent(), use good old devm_kmalloc()
> and issue dma_sync_single_for*() to explicitly switch
> ownership between CPU and the device. This way we exercise the
> cache better and may consume less CPU.
> - Bail out with single segments if we cannot allocate a bounce
> buffer.
> - Tested on the PCI SDHCI on my laptop: requesting a new test
> on i.MX from Benjamin. (Please!)
> ChangeLog v2->v3:
> - Rewrite the commit message a bit
> - Add Benjamin's Tested-by
> - Add Fixes and stable tags
> ChangeLog v1->v2:
> - Skip the remapping and fiddling with the buffer, instead use
> dma_alloc_coherent() and use a simple, coherent bounce buffer.
> - Couple kernel messages to ->parent of the mmc_host as it relates
> to the hardware characteristics.
> ---
> drivers/mmc/host/sdhci.c | 169 ++++++++++++++++++++++++++++++++++++++++++++---
> drivers/mmc/host/sdhci.h | 3 +
> 2 files changed, 164 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
> index e9290a3439d5..51d670b8104d 100644
> --- a/drivers/mmc/host/sdhci.c
> +++ b/drivers/mmc/host/sdhci.c
> @@ -21,6 +21,7 @@
> #include <linux/dma-mapping.h>
> #include <linux/slab.h>
> #include <linux/scatterlist.h>
> +#include <linux/sizes.h>
> #include <linux/swiotlb.h>
> #include <linux/regulator/consumer.h>
> #include <linux/pm_runtime.h>
> @@ -502,8 +503,35 @@ static int sdhci_pre_dma_transfer(struct sdhci_host *host,
> if (data->host_cookie == COOKIE_PRE_MAPPED)
> return data->sg_count;
>
> - sg_count = dma_map_sg(mmc_dev(host->mmc), data->sg, data->sg_len,
> - mmc_get_dma_dir(data));
> + /* Bounce write requests to the bounce buffer */
> + if (host->bounce_buffer) {
> + unsigned int length = data->blksz * data->blocks;
> +
> + if (length > host->bounce_buffer_size) {
> + pr_err("%s: asked for transfer of %u bytes exceeds bounce buffer %u bytes\n",
> + mmc_hostname(host->mmc), length,
> + host->bounce_buffer_size);
> + return -EIO;
> + }
> + if (mmc_get_dma_dir(data) == DMA_TO_DEVICE) {
> + /* Copy the data to the bounce buffer */
> + sg_copy_to_buffer(data->sg, data->sg_len,
> + host->bounce_buffer,
> + length);
> + }
> + /* Switch ownership to the DMA */
> + dma_sync_single_for_device(host->mmc->parent,
> + host->bounce_addr,
> + host->bounce_buffer_size,
> + mmc_get_dma_dir(data));
> + /* Just a dummy value */
> + sg_count = 1;
> + } else {
> + /* Just access the data directly from memory */
> + sg_count = dma_map_sg(mmc_dev(host->mmc),
> + data->sg, data->sg_len,
> + mmc_get_dma_dir(data));
> + }
>
> if (sg_count == 0)
> return -ENOSPC;
> @@ -858,8 +886,13 @@ static void sdhci_prepare_data(struct sdhci_host *host, struct mmc_command *cmd)
> SDHCI_ADMA_ADDRESS_HI);
> } else {
> WARN_ON(sg_cnt != 1);
> - sdhci_writel(host, sg_dma_address(data->sg),
> - SDHCI_DMA_ADDRESS);
> + /* Bounce buffer goes to work */
> + if (host->bounce_buffer)
> + sdhci_writel(host, host->bounce_addr,
> + SDHCI_DMA_ADDRESS);
> + else
> + sdhci_writel(host, sg_dma_address(data->sg),
> + SDHCI_DMA_ADDRESS);
Could use sdhci_sdma_address() here
> }
> }
>
> @@ -2248,7 +2281,12 @@ static void sdhci_pre_req(struct mmc_host *mmc, struct mmc_request *mrq)
>
> mrq->data->host_cookie = COOKIE_UNMAPPED;
>
> - if (host->flags & SDHCI_REQ_USE_DMA)
> + /*
> + * No pre-mapping in the pre hook if we're using the bounce buffer,
> + * for that we would need two bounce buffers since one buffer is
> + * in flight when this is getting called.
> + */
> + if (host->flags & SDHCI_REQ_USE_DMA && !host->bounce_buffer)
> sdhci_pre_dma_transfer(host, mrq->data, COOKIE_PRE_MAPPED);
> }
>
> @@ -2352,8 +2390,45 @@ static bool sdhci_request_done(struct sdhci_host *host)
> struct mmc_data *data = mrq->data;
>
> if (data && data->host_cookie == COOKIE_MAPPED) {
> - dma_unmap_sg(mmc_dev(host->mmc), data->sg, data->sg_len,
> - mmc_get_dma_dir(data));
> + if (host->bounce_buffer) {
> + /*
> + * On reads, copy the bounced data into the
> + * sglist
> + */
> + if (mmc_get_dma_dir(data) == DMA_FROM_DEVICE) {
> + unsigned int length = data->bytes_xfered;
> +
> + if (length > host->bounce_buffer_size) {
> + pr_err("%s: bounce buffer is %u bytes but DMA claims to have transferred %u bytes\n",
> + mmc_hostname(host->mmc),
> + host->bounce_buffer_size,
> + data->bytes_xfered);
> + /* Cap it down and continue */
> + length = host->bounce_buffer_size;
> + }
> + dma_sync_single_for_cpu(
> + host->mmc->parent,
> + host->bounce_addr,
> + host->bounce_buffer_size,
> + DMA_FROM_DEVICE);
> + sg_copy_from_buffer(data->sg,
> + data->sg_len,
> + host->bounce_buffer,
> + length);
> + } else {
> + /* No copying, just switch ownership */
> + dma_sync_single_for_cpu(
> + host->mmc->parent,
> + host->bounce_addr,
> + host->bounce_buffer_size,
> + mmc_get_dma_dir(data));
> + }
> + } else {
> + /* Unmap the raw data */
> + dma_unmap_sg(mmc_dev(host->mmc), data->sg,
> + data->sg_len,
> + mmc_get_dma_dir(data));
> + }
> data->host_cookie = COOKIE_UNMAPPED;
> }
> }
> @@ -2543,6 +2618,14 @@ static void sdhci_adma_show_error(struct sdhci_host *host)
> }
> }
>
> +static u32 sdhci_sdma_address(struct sdhci_host *host)
> +{
> + if (host->bounce_buffer)
> + return host->bounce_addr;
> + else
> + return sg_dma_address(host->data->sg);
> +}
> +
> static void sdhci_data_irq(struct sdhci_host *host, u32 intmask)
> {
> u32 command;
> @@ -2636,7 +2719,8 @@ static void sdhci_data_irq(struct sdhci_host *host, u32 intmask)
> */
> if (intmask & SDHCI_INT_DMA_END) {
> u32 dmastart, dmanow;
> - dmastart = sg_dma_address(host->data->sg);
> +
> + dmastart = sdhci_sdma_address(host);
> dmanow = dmastart + host->data->bytes_xfered;
> /*
> * Force update to the next DMA block boundary.
> @@ -3217,6 +3301,68 @@ void __sdhci_read_caps(struct sdhci_host *host, u16 *ver, u32 *caps, u32 *caps1)
> }
> EXPORT_SYMBOL_GPL(__sdhci_read_caps);
>
> +static int sdhci_allocate_bounce_buffer(struct sdhci_host *host)
> +{
> + struct mmc_host *mmc = host->mmc;
> + unsigned int max_blocks;
> + unsigned int bounce_size;
> + int ret;
> +
> + /*
> + * Cap the bounce buffer at 64KB. Using a bigger bounce buffer
> + * has diminishing returns, this is probably because SD/MMC
> + * cards are usually optimized to handle this size of requests.
> + */
> + bounce_size = SZ_64K;
> + /*
> + * Adjust downwards to maximum request size if this is less
> + * than our segment size, else hammer down the maximum
> + * request size to the maximum buffer size.
> + */
> + if (mmc->max_req_size < bounce_size)
> + bounce_size = mmc->max_req_size;
> + max_blocks = bounce_size / 512;
> +
> + /*
> + * When we just support one segment, we can get significant
> + * speedups by the help of a bounce buffer to group scattered
> + * reads/writes together.
> + */
> + host->bounce_buffer = devm_kmalloc(mmc->parent,
> + bounce_size,
> + GFP_KERNEL);
> + if (!host->bounce_buffer) {
> + pr_err("%s: failed to allocate %u bytes for bounce buffer, falling back to single segments\n",
> + mmc_hostname(mmc),
> + bounce_size);
> + /*
> + * Exiting with zero here makes sure we proceed with
> + * mmc->max_segs == 1.
> + */
> + return 0;
> + }
> +
> + host->bounce_addr = dma_map_single(mmc->parent,
> + host->bounce_buffer,
> + bounce_size,
> + DMA_BIDIRECTIONAL);
> + ret = dma_mapping_error(mmc->parent, host->bounce_addr);
> + if (ret)
> + /* Again fall back to max_segs == 1 */
> + return 0;
> + host->bounce_buffer_size = bounce_size;
> +
> + /* Lie about this since we're bouncing */
> + mmc->max_segs = max_blocks;
> + mmc->max_seg_size = bounce_size;
> + mmc->max_req_size = bounce_size;
> +
> + pr_info("%s bounce up to %u segments into one, max segment size %u bytes\n",
> + mmc_hostname(mmc), max_blocks, bounce_size);
> +
> + return 0;
> +}
> +
> int sdhci_setup_host(struct sdhci_host *host)
> {
> struct mmc_host *mmc;
> @@ -3713,6 +3859,13 @@ int sdhci_setup_host(struct sdhci_host *host)
> */
> mmc->max_blk_count = (host->quirks & SDHCI_QUIRK_NO_MULTIBLOCK) ? 1 : 65535;
>
> + if (mmc->max_segs == 1) {
> + /* This may alter mmc->*_blk_* parameters */
> + ret = sdhci_allocate_bounce_buffer(host);
> + if (ret)
> + return ret;
> + }
> +
> return 0;
>
> unreg:
> diff --git a/drivers/mmc/host/sdhci.h b/drivers/mmc/host/sdhci.h
> index 54bc444c317f..1d7d61e25dbf 100644
> --- a/drivers/mmc/host/sdhci.h
> +++ b/drivers/mmc/host/sdhci.h
> @@ -440,6 +440,9 @@ struct sdhci_host {
>
> int irq; /* Device IRQ */
> void __iomem *ioaddr; /* Mapped address */
> + char *bounce_buffer; /* For packing SDMA reads/writes */
> + dma_addr_t bounce_addr;
> + unsigned int bounce_buffer_size;
>
> const struct sdhci_ops *ops; /* Low level hw interface */
>
>