Re: [RFC/PATCHv2 3/3] spi: dw-spi: Pointers select 16b vs. 32b DesignWare access

[Thor Thayer <tthayer-yzvPICuk2ABMcg4IHK0kFoH6Mc4MB0Vx@public.gmane.org>]

On 03/09/2015 03:02 PM, Andy Shevchenko wrote:
> On Mon, Mar 9, 2015 at 9:47 PM, Thor Thayer
> <tthayer-yzvPICuk2ABMcg4IHK0kFoH6Mc4MB0Vx@public.gmane.org> wrote:
>> On 03/09/2015 01:54 PM, Andy Shevchenko wrote:
>
>> Yes, I just need the 32 bit write. I was trying to remain consistent but I
>> agree that only changing only writes would minimize the changes.
>
> Which is still makes me anxious.
>
> I have briefly read a chapter for SPI in pdf you sent link to. I
> didn't find anything except SPI supports 32 bit data width.
>
> It would be nice if you ping your HW engineers and clarify what
> exactly is happening there.
>

I confirmed with our HW engineer that writes are required to be 32 bits. 
We are in the process of updating our documentation to explicitly say this.

Since there are no byte enables on our 32 bit APB interface, writing 
only 1 or 2 bytes could corrupt the other bytes in the 32 bit word. The 
32 bit write enforces requiring an explicit read/modify/write for less 
than 32 bit writes.

Since reads are non-destructive (nothing is being written back into the 
register), it is safe for all reads to get promoted to 32 bit reads.

Is it preferable to have both 32 bit reads and writes for consistency?

Or is it preferable to make the minimal number of changes which would be 
to only add a 32 bit write function?
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