Re: [PATCH 05/10] spi: bcm2835: Work around DONE bit erratum

[Stefan Wahren <wahrenst@gmx.net>]

Am 11.08.19 um 21:57 schrieb Lukas Wunner:
> 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 thought about document this issue in some kind of errata or on their
website.
>
> 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.
>
> Thanks,
>
> Lukas