Re: [RFC-PATCH] mtd: spi-nor: add conditional 4B opcodes

[Daniel Walker (danielwa)]

On Sat, May 09, 2020 at 12:37:35AM +0530, Pratyush Yadav wrote:
> Hi Daniel,
> 
> On 07/05/20 06:13PM, Daniel Walker (danielwa) wrote:
> > On Thu, May 07, 2020 at 11:33:46PM +0530, Pratyush Yadav wrote:
> > > On 07/05/20 09:20AM, Daniel Walker wrote:
> > > > Some chips have 4B opcodes, but there is no way to know if they have
> > > > them. This device tree option allows platform owners to force enable 4b
> > > > opcodes when they know their chips support it even when it can be
> > > > automatically identified.
> > > 
> > > Do you mean that two chips might have the same ID but one of them can 
> > > support 4B opcodes and the other can not? Is it possible to detect this 
> > > in a fixup hook? I think it would be better to do something like this in 
> > > a fixup hook instead of via device tree.
> >   
> > Yes. The chip I added the option for is an example of this, it's n25q256a. I'm not familiar with the
> > fixup hook mechanism, but I would assume you need some way to tell between the 4B
> > opcode chips and the non-4B opcode chips. For n25q256a, we have not found a way
> > to do that.
> 
> I'm assuming this patch is related to [0]. If all you want is to address 
> memory above 16M, why not switch to 4-byte addressing mode instead? 
> Taking a quick look at the datasheet tells me this can be done via the 
> "Enter 4-byte address mode" command (0xB7). Then just use the regular 
> read/program commands with 4-byte addresses. Does that work for you? Is 
> there any reason you _have_ to use dedicated 4B opcodes?

It might, I don't think we need anything beyond access to move than 16M. Your
proposal would be to have a hook which enters the 0xB7 command?

I guess the question would be do all the chips have this ability.

Daniel