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

[Lukas Wunner <lukas@xxxxxxxxx>]

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.

Thanks,

Lukas