Re: [virtio-comment] [PATCH] [PATCH v5] virtio-spi: add the device specification

[Haixu Cui <quic_haixcui@quicinc.com>]

On 11/29/2023 4:33 PM, Cornelia Huck wrote:
> On Wed, Nov 29 2023, Haixu Cui <quic_haixcui@quicinc.com> wrote:
>
> > Hi Viresh,
> >
> > On 11/29/2023 3:30 PM, Viresh Kumar wrote:
> > > On 28-11-23, 20:58, Haixu Cui wrote:
> > > > On 11/27/2023 6:17 PM, Viresh Kumar wrote:
> > > > > On 24-11-23, 15:20, Haixu Cui wrote:
> > > > > > +For half-duplex read transfer, \field{rx_buf} is filled by Virtio SPI device and consumed
> > > > > > +by Virtio SPI driver. For half-duplex write transfer, \field{tx_buf} is filled by Virtio
> > > > > > +SPI driver and consumed by Virtio SPI device. And for full-duplex read and write transfer,
> > > > > > +both \field{tx_buf} and \field{rx_buf} are used.
> > > > >
> > > > > Should the length of both the buffers in full-duplex mode be same ? If yes, then
> > > > > this should be mentioned (in case it is not).
> > > >
> > > > No, there is no such limitation. Write and read buffers may be different is
> > > > size.
> > >
> > > Hmm, I worked with a SPI controller over a decade ago, and I must be forgetting
> > > something here I guess. But from whatever little I remember, with full-duplex
> > > transfer, data flows on both MOSI and MISO lines as soon as clock signal is
> > > applied.  And so amount of data sent is always be equal to amount of data
> > > received by both sides.
> > >
> > > Also if I see Linux's implementation of the `struct spi_transfer` [1], I see
> > > `tx_buf`, `rx_buf` and a single `len` field, which applies to both the buffers.
> > > Which I guess is indicating that both buffers are supposed to be of same length.
> > >
> > > What am I missing ?
> >
> > Oh so sorry for that. And I don't make it clear. Yes, tx_buf and rx_buf
> > have the same size, Linux has such restriction. Just as you mention,
> > kernel level spi_transfer has single "len", the same for
> > spi_ioc_transfer passed from the userland.
> >
> > But I am not sure if this is in the scope of the spec. Because this is
> > ensured by Linux, but Virtio SPI driver won't also can't verify this.
> > This is a prerequisite for virtio spi processing requests.
> >
> > What is your suggestion? How about adding some descriptions here, like
> > "for full-duplex, tx_buf and rx_buf are same in size, this is guaranteed
> > by the kernel"?
>
> We must not really make any assumptions in the spec about concrete
> implementations (here, the Linux kernel), as someone implementing it in
> a different environment will need to make explicit choices.
>
> So, if tx_buf and rx_buf are required to be of the same size, it needs
> to be explicitly stated in the spec, or an implementation might choose
> to do it differently.

Okay! Thanks Huck for guidance and thanks Viresh for pointing it out. 
Much appreciated.

Best Regards
Haixu Cui

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