linux-kernel - Re: [PATCH v2 3/4] spi: Split transfers larger than max size

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Message-ID: <ZJS6OnzRapPCboC8@finisterre.sirena.org.uk>
Date:   Thu, 22 Jun 2023 22:16:42 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Eddie James <eajames@...ux.ibm.com>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        krzysztof.kozlowski@...aro.org, andi@...zian.org, kernel@...s.com,
        alim.akhtar@...sung.com, linux-spi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 3/4] spi: Split transfers larger than max size

On Thu, Jun 22, 2023 at 02:48:36PM -0500, Eddie James wrote:
> On 9/27/22 06:21, Vincent Whitchurch wrote:
> > A couple of drivers call spi_split_transfers_maxsize() from their
> > ->prepare_message() callbacks to split transfers which are too big for
> > them to handle.  Add support in the core to do this based on
> > ->max_transfer_size() to avoid code duplication.

> I've been testing AT25 functionality in linux 6.1 and I believe this patch
> is breaking the AT25 protocol. It will split a write command up such that
> some of the data is in a different transfer than  the write enable and
> address. According to my understanding of the AT25 spec, that doesn't
> work... Someone correct me if I'm wrong though. Do we need a flag to
> enable/disable this behavior depending on the client perhaps?

Could you be more specific about the manner in which you think this is
breaking things?  The size of transfer is immaterial to the client
device on SPI, the client will be counting clocks while the chip select
is asserted.  How the controller chooses to split things up is really
not particularly visible or relevant, it might bitbang things out one
bit at a time, transfer a single word at a time or batch things up
further.  So long as the chip select is asserted it's all the same to
the client device.

In any case this is all based on the maximum transfer size advertised by
the conteroller driver, if the device can physically handle larger
transfers then there's no reason for it to set a limit.  If the driver
can't physically handle larger transfers and it does make a difference
then the system simply won't work.

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