Re: [PATCH 1/2] mtd: spi-nor: add an option to force 3byte adressing mode

[Marek Vasut <marek.vasut@xxxxxxxxx>]

On 08/28/2017 11:14 PM, Marcin Wojtas wrote:
> Hitherto code set 4B addressing mode for all SPI flashes whose
> size exceeds 16MB. However, changing the default 3B access
> in some cases may be harmful - it may happen that the Boot ROM
> is not capable of handling non-default state of the SPI NOR
> (e.g. after soft reset).

No, this happens when your hardware is broken and does NOT reset the
flash while reseting the CPU as well. If the flash is in some funny
state (ie. middle of page program or erase cycle, which takes long OR
4byte addressing mode) and the CPU resets , then if the SPI NOR is not
reset, your system will still get stuck . You need to fix your hardware
such that you use the reset output of your CPU to detect when the CPU
restarted and add logic to reset your storage as well.

This stuff below is a poor workaround and has it's known problems which
can only be solved by fixing the hardware. This problem keeps coming up
repeatedly, just skim through the Linux MTD and U-Boot lists for other
victims of bad HW design. I don't think I want to see it in the kernel
as that would motivate people to use it rather than fixing their HW .

> Some flash devices allow to access the
> memory above 128Mib without changing mode to 4byte thanks
> to special op codes (see SPI_NOR_4B_OPCODES flag). Unfortunately
> for those which don't support them, the problem persists.
> 
> This patch adds optional property that can be added to the
> SPI flash node and which will force to use 3B addressing mode,
> limiting the accessible memory size to 16MiB.
> Binding documentation is updated accordingly.
> 
> Signed-off-by: Marcin Wojtas <mw@xxxxxxxxxxxx>
> ---
>  Documentation/devicetree/bindings/spi/spi-bus.txt |  2 ++
>  drivers/mtd/spi-nor/spi-nor.c                     | 12 +++++++++++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/devicetree/bindings/spi/spi-bus.txt b/Documentation/devicetree/bindings/spi/spi-bus.txt
> index 1f6e86f..f13b773 100644
> --- a/Documentation/devicetree/bindings/spi/spi-bus.txt
> +++ b/Documentation/devicetree/bindings/spi/spi-bus.txt
> @@ -77,6 +77,8 @@ All slave nodes can contain the following optional properties:
>  		    Defaults to 1 if not present.
>  - spi-rx-delay-us - Microsecond delay after a read transfer.
>  - spi-tx-delay-us - Microsecond delay after a write transfer.
> +- spi-3byte-addressing - Empty property indicating device access to be done
> +		    only in 3byte addressing mode.
>  
>  Some SPI controllers and devices support Dual and Quad SPI transfer mode.
>  It allows data in the SPI system to be transferred using 2 wires (DUAL) or 4
> diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
> index 1413828..029c87d 100644
> --- a/drivers/mtd/spi-nor/spi-nor.c
> +++ b/drivers/mtd/spi-nor/spi-nor.c
> @@ -2002,7 +2002,17 @@ int spi_nor_scan(struct spi_nor *nor, const char *name,
>  		if (JEDEC_MFR(info) == SNOR_MFR_SPANSION ||
>  		    info->flags & SPI_NOR_4B_OPCODES)
>  			spi_nor_set_4byte_opcodes(nor, info);
> -		else
> +		else if (of_property_read_bool(np, "spi-3byte-addressing")) {
> +			/*
> +			 * Do not enter 4byte mode in order to prevent
> +			 * the early bootloader to come up on non-default
> +			 * SPI NOR memory during boot. Limit accessible
> +			 * size to 16MiB.
> +			 */
> +			nor->addr_width = 3;
> +			mtd->size = 0x1000000;
> +			dev_info(dev, "Force 3B addressing mode\n");
> +		} else
>  			set_4byte(nor, info, 1);
>  	} else {
>  		nor->addr_width = 3;
> 


-- 
Best regards,
Marek Vasut
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