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

[Stefan Wahren <wahrenst@xxxxxxx>]

Hi Lukas,

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?