RfC: Handle SPI controller limitations like maximum message length

[Heiner Kallweit]

Am 20.11.2015 um 13:35 schrieb Mark Brown:
> On Fri, Nov 20, 2015 at 11:06:47AM +0100, Heiner Kallweit wrote:
>> On Fri, Nov 20, 2015 at 7:59 AM, Heiner Kallweit <hkallweit1 at gmail.com> wrote:
> 
>>> 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.
> 
> We can't assume that users of spi_write_then_read() will cope with a
> restarted transfer - the usual use case is things like register I/O
> where restarting a partial transfer wouldn't produce the desired result
> so it's just a plain error for users of that interface.  Anything that
> is able to cope needs to be using the core API directly.
> 
>> 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.
> 
> You'll get an error on truncation.  It may be possible to recover.
> 
OK, I interpret this as:
Controller drivers shall return 0 only if the complete message was
transferred successfully.
If  a controller driver returns an error it has the option to set
actual_length to what was transferred successfully.

This means we can't use patch 4 from Michal because it bails out as soon
as the underlying SPI transfer returns an error.

Instead something like the spi-nor patch I sent on Oct 6th would be needed:
[PATCH] mtd: spi-nor: handle controller driver limitations in spi_nor_read

It loops over nor->read and ignores errors as long as at least something
was read.