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Subject: Re: [virtio-comment] [PATCH] [PATCH v5] virtio-spi: add the device specification
On 11/29/2023 6:00 PM, Viresh Kumar wrote:
On 29-11-23, 16:54, Haixu Cui wrote:On 11/29/2023 4:33 PM, Cornelia Huck wrote:On Wed, Nov 29 2023, Haixu Cui <quic_haixcui@quicinc.com> wrote: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: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.I don't think this is a Linux only thing. This is how the SPI protocol is designed to begin with. A clock is applied, data from FIFOs of both master and slaves gets exchanged over the MOSI and MISO lines... And the length is _always_ the same. We are talking about the full-duplex mode of the hardware here and this is how it works AFAICT. So the spec should clearly point this out to avoid any mistakes. Or if you have a usecase where you can have different lengths for tx and rx buffers, that I am not aware of ?
Hi Viresh,Thank you for your helpful comments. In next patch, I will clearly point this out:
"For full-duplex read and write transfer, both \field{tx_buf} and \field{rx_buf} are used and the buffer size of \field{tx_buf} must be same as \field{rx_buf}."
And in drivernormative section, I will add a requirement:"For full-duplex transfer, Virtio SPI driver MUST guarantee the write transfer size is equal to the read transfer size"
In fact, in my prototype, I only perform the transfers with same tx/rx length.
Much appreciated. Best Regards Haixu Cui
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.I agree. I pointed this out since this is how the protocol works.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.-- Viresh
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