Re: [PATCH 0/2] spi: fsi: Reduce max transfer size to 8 bytes

On Sat, 2021-07-17 at 13:46 +0000, David Laight wrote:

From: Eddie James

quoted

Sent: 16 July 2021 14:39

The security restrictions on the FSI-attached SPI controllers have
been applied universally to all controllers, so the controller can
no
longer transfer more than 8 bytes for one transfer. Refactor the
driver
to remove the looping and support for larger transfers, and remove
the
"restricted" compatible string, as all the controllers are now
considered restricted.

Aren't there significant performance (and device wear?) penalties
for doing short SPI eeprom writes?

Probably so. However there is no choice in the matter, as the SPI
controller can't process larger reads/writes.
(I should note that the controller/driver CAN process up to 40 byte
writes. However, the driver must report the minimum of read and write
sizes in the max_transfer_size callback, so no client driver should
request larger than the max read size - 8 bytes).

Thanks,
Eddie

IIRC (and I'm not getting my serial busses confused) a write request
can request an aligned transfer of up to (typically) 32 bytes.
At which point you need to wait for the status to indicate
'complete'.

So restricting writes to 8 bytes increases block write times
by a factor of 4.

Increasing the numbers of writes by a factor or 4 may also have
an effect on device wear - but that is more likely only affected
by erase cycles.

	David

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