Re: [PATCH] mtd: spi-nor: honour max_data_size for spi-nor writes

[NeilBrown]

On Mon, Jun 11 2018, Boris Brezillon wrote:
>
> Also, I'd prefer to have this patch split in 2:
> 1/ one patch removing the check in spi_nor_write()
> 2/ and the second patch removing the while() loop in m25p80_write()
>
> How about the following commit messages for those 2 patches:
>
> 1:
> "
> mtd: spi-nor: Support controllers with limited TX FIFO size
>
> Some SPI controllers can't write nor->page_size bytes in a single step
> because their TX FIFO is too small.

I no longer think this is good justification for changes to m25p80 or to
anything outside the low-level SPI driver.  The size of the FIFO is not
related to the maximum message size.

An SPI transaction involves:
 - asserting chip-select
 - sending/receiving a bunch of bits
 - deasserting chip-select

That middle part of sending/receiving bits can be done:
 - one bit at a time by bit-banging GPIOs
 - 32 bytes at a time by filling a buffer, running the SPI engine, then
    reading results out of the buffer (assuming a 32-byte buffer)
 - with one single DMA operation if the SPI engine can DMA to/from
   main memory

and there are probably other options.

If your SPI engine only handles (say) 32 bytes at a time, then call it
repeatedly, while holding chip-select asserted the whole time.  The SPI
clock is controlled by the host and in this case it will not have a
stable frequency for the whole transaction (occasionally pauses), but
that doesn't matter.  The slave just watches for the clock transitions
and don't care when they come as long as they don't come too fast.

I recently modified the mt7621 spi driver (in drivers/staging) to work
like this and got much better throughput when reading from spi-nor flash
memory.  It handles writes of full pages (36 bytes at at time!) without
problems.

Thanks,
NeilBrown

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