Lukas Wunner <lukas@xxxxxxxxx> writes:
> On Sun, Aug 11, 2019 at 09:45:09PM +0200, Stefan Wahren wrote:
>> Am 03.08.19 um 12:10 schrieb Lukas Wunner:
>> > Commit 3bd7f6589f67 ("spi: bcm2835: Overcome sglist entry length
>> > limitation") amended the BCM2835 SPI driver with support for DMA
>> > transfers whose buffers are not aligned to 4 bytes and require more than
>> > one sglist entry.
>> >
>> > When testing this feature with upcoming commits to speed up TX-only and
>> > RX-only transfers, I noticed that SPI transmission sometimes breaks.
>> > A function introduced by the commit, bcm2835_spi_transfer_prologue(),
>> > performs one or two PIO transmissions as a prologue to the actual DMA
>> > transmission. It turns out that the breakage goes away if the DONE bit
>> > in the CS register is set when ending such a PIO transmission.
>> >
>> > The DONE bit signifies emptiness of the TX FIFO. According to the spec,
>> > the bit is of type RO, so writing it should never have any effect.
>> > Perhaps the spec is wrong and the bit is actually of type RW1C.
>> > E.g. the I2C controller on the BCM2835 does have an RW1C DONE bit which
>> > needs to be cleared by the driver. Another, possibly more likely
>> > explanation is that it's a hardware erratum since the issue does not
>> > occur consistently.
>> >
>> > Either way, amend bcm2835_spi_transfer_prologue() to always write the
>> > DONE bit.
>> >
>> > Usually a transmission is ended by bcm2835_spi_reset_hw(). If the
>> > transmission was successful, the TX FIFO is empty and thus the DONE bit
>> > is set when bcm2835_spi_reset_hw() reads the CS register. The bit is
>> > then written back to the register, so we happen to do the right thing.
>> >
>> > However if DONE is not set, e.g. because transmission is aborted with
>> > a non-empty TX FIFO, the bit won't be written by bcm2835_spi_reset_hw()
>> > and it seems possible that transmission might subsequently break. To be
>> > on the safe side, likewise amend bcm2835_spi_reset_hw() to always write
>> > the bit.
>>
>> has the issue already reported to Raspberry Pi Trading?
>
> You mean to fix such errata in future revisions?
>
> I wouldn't know who to report this to, Roger Thornton or James Adams perhaps?
>
> I'm not sure if the SPI controller isn't just an IP block licensed from
> a third party, that might make it difficult to get errata fixed.
It's Broadcom's.
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