linux-kernel - Re: [PATCH RFC 1/3] spi: Allow SPI controller override device buswidth

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Date:   Tue, 3 Mar 2020 09:42:45 +0000
From:   John Garry <john.garry@...wei.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Mark Brown <broonie@...nel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        MTD Maling List <linux-mtd@...ts.infradead.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH RFC 1/3] spi: Allow SPI controller override device
 buswidth

Hi Geert,

> 
> On Fri, Feb 28, 2020 at 4:23 PM John Garry <john.garry@...wei.com> wrote:
>> Currently ACPI firmware description for a SPI device does not have any
>> method to describe the data buswidth on the board.
>>
>> So even through the controller and device may support higher modes than
>> standard SPI, it cannot be assumed that the board does - as such, that
>> device is limited to standard SPI in such a circumstance.
>>
>> As a workaround, allow the controller driver supply buswidth override bits,
>> which are used inform the core code that the controller driver knows the
>> buswidth supported on that board for that device.
> 
> Just wondering: can't the controller just override this (e.g. in the .setuup()
> callback) without having to touch the generic code?

I think that this is a good idea.

However, where we call .setup() in spi_setup() looks a little too late - 
at the point we call .setup(), most of the work on vetting/fixing the 
mode bits is complete. And I would not want the SPI controller driver 
just to disregard this work and overwrite the bits (in this way).

And if we wanted to move the .setup callback earlier, then I would be 
worried here with 2 things:
1. Some SPI controller drivers may rely on spi->mode being set finally 
when .setup() is called
2. We may need to undo the work of .setup() if we later error in 
spi_setup(), like for invalid mode bits

However, maybe another callback could be introduced, .early_setup().

Thanks,
John