[PATCH][RFC] mtd: spinand: fix detection of GD5FxGQ4xA flash

Chuanhong Guo gch981213 at gmail.com
Sun Nov 3 04:03:21 PST 2019


On Tue, Oct 29, 2019 at 12:41 AM Miquel Raynal
<miquel.raynal at bootlin.com> wrote:
> Hello,
> Chuanhong Guo <gch981213 at gmail.com> wrote on Wed, 16 Oct 2019 09:38:24
> +0800:
> > GD5FxGQ4xA didn't follow the SPI spec to keep MISO low while slave is
> > reading, and instead MISO is kept high. As a result, the first byte
> > of id becomes 0xFF.
> > Since the first byte isn't supposed to be checked at all, this patch
> > just removed that check.
> >
> > While at it, redo the comment above to better explain what's happening.
> >
> > Fixes: cfd93d7c908e ("mtd: spinand: Add support for GigaDevice GD5F1GQ4UFxxG")
> > Signed-off-by: Chuanhong Guo <gch981213 at gmail.com>
> > CC: Jeff Kletsky <git-commits at allycomm.com>
> > ---
> > RFC:
> > I doubt whether this patch is a proper fix for the underlying problem:
> > The actual problem is that we have two different implementation of read id
> > command: One replies immediately after master sending 0x9f and the other
> > need to send 0x9f and an offset byte (found in winbond and early GD flashes.)

Correction: Only early GigaDevice nand chips uses this implementation.
Winbond chips uses a dummy byte instead of an address byte so there's
no problem for Winbond chips.

> > Current code only works if SPI master is properly implemented (i.e. keep MOSI
> > low while reading.)
> I am not entirely against the fix, but this is a SPI host controller
> issue, right? Can you try to fix the controller driver instead?

I think this is a spi nand framework issue. GigaDevice uses an unusual
READ ID implementation, and as a result, both host controller and chip
are reading during the first byte after 0x9f command: chip is reading
the address/offset byte and host is expecting the first ID byte.
Here lies two problems:
1. According to the sequence diagram in their datasheet, MISO pin is
in High-Z state during the 0x9f command and the offset byte, and host
could read anything during this time instead of a fixed 0x0 or 0xff
byte, so the check of first byte should be removed. This is what this
patch is doing.
2. If there's a buggy SPI host controller that didn't keep MOSI low
during reading operation, the chip will get 0xff as ID offset, causing
the read vendor/device ID to be swapped. I never met such a controller
so far, but if there is one, it will be a silicon bug that can't be
fixed by software. To fix this one, we'll have to make a second
read-id implementation in spi nand framework.

The second problem only exist in theory, so my preference is to apply
this patch and fix only the first problem for now.

Chuanhong Guo

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