RfC: Handle SPI controller limitations like maximum message length

[Heiner Kallweit]

On Fri, Nov 20, 2015 at 7:59 AM, Heiner Kallweit <hkallweit1 at gmail.com> wrote:
> Am 20.11.2015 um 01:02 schrieb Brian Norris:
>> On Wed, Nov 18, 2015 at 10:19:29PM +0100, Heiner Kallweit wrote:
>>> There have been few discussions in the past about how to handle SPI controller
>>> limitations like max message length. However they don't seem to have resulted
>>> in accepted patches yet.
>>> I also stumbled across this topic because I own a device using Freescale's
>>> ESPI which has a 64K message size limitation.
>>>
>>> At least one agreed fact is that silently assembling chunks in protocol
>>> drivers is not the preferred approach.
>>
>> Hmm, are you referring to this sort of approach [1], where the
>> spi_message::acutal_length informs the spi_nor layer that the transfer
>> was truncated?
>>
>> [1] [PATCH v4 7/7] mtd: spi-nor: add read loop
>>     http://lists.infradead.org/pipermail/linux-mtd/2015-August/061062.html
>>
>>> Maybe a better approach would be to introduce a new member of spi_master
>>> dealing with controller limitations.
>>> My issue is just the message size limitation but most likely there are more
>>> and different limitations in other controllers.
>>>
>>> I'd introduce a struct spi_controller_restrictions and add a member to spi_master
>>> pointing to such a struct. Then a controller driver could do something like this:
>>>
>>> static const struct spi_controller_restrictions fsl_espi_restrictions = {
>>>      .max_msg_size   = 0xffff,
>>> };
>>>
>>> master->restrictions = &fsl_espi_restrictions;
>>
>> OK, so I think Mark suggested we not move to a 'restrictions' struct,
>> but otherwise it doesn't sound like he's opposed to this.
>>
> That's how I read his comments too.
>
>>> I also add an example how a protocol driver could use this extension.
>>> Appreciate any comment.
>>
>> One question I have: is it necessary to push the handling out into the
>> protocol driver? I feel like I've seen a partial answer to this: the
>> 'actual_legnth' return field suggests that the protocol driver already
>> has to deal with shorter-than-desired transfers.
>>
>> Then I have another one: is the 'actual_length' field really
>> insufficient? For instance, is it non-kosher for a spi_master to just
>> cutoff the message at (for instance) 64K, and expect the protocol
>> driver to handle that (e.g., with Michal's patch from [1])? And if that
>> is kosher, then is there a good reason for the protocol driver to know
>> the exact maximum for its spi_master?
>>
> It would be sufficient if it's a valid case that spi_master returns 0
> and an actual_length < requested_length as this is some kind of error
> situation.

I had one more look at the SPI core and e.g. spi_write_then_read
calls spi_sync w/o checking actual_length afterwards.
This can mean the discussed case is not valid, however it also could be
simply a bug.

If the discussed case is valid a clear hint to all users of spi_sync and
friends should be added that the caller can not rely on status code 0
only but must check actual_length to verify that the complete message
was transferred.

It would be good to hear Mark's opinion on this.

> I could also fully understand if spi_master doesn't return 0 but
> -EMSGSIZE in such a case.
> And the suggested patch would bail out of the chunk-assembling loop
> once it get's an error from the SPI transfer
> (after applying patch 2 of the series which introduces checking
> the return code of the spi_sync call in m25p80_read).
>
>> [snip example]
>>
>> Brian
>>
>