linux-kernel - Re: [RFC PATCH v3 3/3] SPI: Add virtio SPI driver.

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Message-ID: <f02f1784-123d-44f8-afca-63a593c9d1da@sirena.org.uk>
Date: Tue, 27 Feb 2024 14:25:03 +0000
From: Mark Brown <broonie@...nel.org>
To: Harald Mommer <harald.mommer@...nsynergy.com>
Cc: virtio-dev@...ts.oasis-open.org, Haixu Cui <quic_haixcui@...cinc.com>,
	Viresh Kumar <viresh.kumar@...aro.org>, linux-spi@...r.kernel.org,
	linux-kernel@...r.kernel.org, quic_ztu@...cinc.com,
	Matti Moell <Matti.Moell@...nsynergy.com>,
	Mikhail Golubev <Mikhail.Golubev@...nsynergy.com>
Subject: Re: [RFC PATCH v3 3/3] SPI: Add virtio SPI driver.

On Tue, Feb 27, 2024 at 03:12:55PM +0100, Harald Mommer wrote:
> On 13.02.24 18:49, Mark Brown wrote:
> > On Tue, Feb 13, 2024 at 02:53:50PM +0100, Harald Mommer wrote:

> > > +static int virtio_spi_one_transfer(struct virtio_spi_req *spi_req,
> > > +				   struct spi_controller *ctrl,
> > > +				   struct spi_message *msg,
> > > +				   struct spi_transfer *xfer)
> > > +	/*
> > > +	 * Got comment: "The virtio spec for cs_change is *not* what the Linux
> > > +	 * cs_change field does, this will not do the right thing."
> > > +	 * TODO: Understand/discuss this, still unclear what may be wrong here
> > > +	 */

> > > +	th->cs_change = xfer->cs_change;

> I got the comment originally from you, Mark Brown. After some digging still
> unclear what should be wrong and in the meantime I think nothing is wrong at
> all. To point you with the nose on the pending issue I added this comment
> here.

Without going and checking the spec IIRC cs_change only applies within a
message in the virtio spec but it has effects on the final transfer in
Linux.  

> 
> I'll remove the comment because I think there is no problem. Please protest
> if I'm wrong.
> 
> > > +static int virtio_spi_transfer_one_message(struct spi_controller *ctrl,
> > > +					   struct spi_message *msg)
> > > +{
> > > +	struct virtio_spi_priv *priv = spi_controller_get_devdata(ctrl);
> > > +	struct virtio_spi_req *spi_req;
> > > +	struct spi_transfer *xfer;
> > > +	int ret = 0;
> > > +
> > > +	spi_req = kzalloc(sizeof(*spi_req), GFP_KERNEL);
> > > +	if (!spi_req) {
> > > +		ret = -ENOMEM;
> > > +		goto no_mem;
> > > +	}
> > Why not just allocate this once, it's not like it's possible to send
> > more than one message simultaneously?
> Will be done, struct virtio_spi_req spi_req will become a member of struct
> virtio_spi_priv.
> > > +	/*
> > > +	 * Simple implementation: Process message by message and wait for each
> > > +	 * message to be completed by the device side.
> > > +	 */
> > > +	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
> > This is processing transfers within a message rather than messages.
> Obviously I did not get the terminology of messages and transfers not
> correct which made the comment wrong. Comment to be corrected (and
> shortened).
> > > +		ret = virtio_spi_one_transfer(spi_req, ctrl, msg, xfer);
> > > +		if (ret)
> > > +			goto msg_done;
> > > +
> > > +		virtqueue_kick(priv->vq);
> > > +
> > > +		wait_for_completion(&spi_req->completion);
> > > +
> > > +		/* Read result from message */
> > > +		ret = (int)spi_req->result.result;
> > > +		if (ret)
> > > +			goto msg_done;
> > It's not clear why this isn't within _spi_transfer_one() and then we
> > don't just use a transfer_one() callback and factor everything out to
> > the core?
> 
> Lack of experience. I saw one way of doing the job which missing the more
> simple way. Therefore we have reviews. Using now the alternative callback
> which shortens and simplifies the code.
> 
> Applied code changes, have to run some more tests.
> 
> 

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