linux-kernel - Re: [PATCH v2 0/5] mtd: spi-nor: Add support for Octal 8D-8D-8D mode

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Date:   Tue, 5 May 2020 11:44:43 +0200
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     masonccyang@...c.com.tw
Cc:     "Pratyush Yadav" <me@...avpratyush.com>, broonie@...nel.org,
        juliensu@...c.com.tw, linux-kernel@...r.kernel.org,
        linux-mtd@...ts.infradead.org, linux-spi@...r.kernel.org,
        miquel.raynal@...tlin.com, "Pratyush Yadav" <p.yadav@...com>,
        richard@....at, tudor.ambarus@...rochip.com, vigneshr@...com
Subject: Re: [PATCH v2 0/5] mtd: spi-nor: Add support for Octal 8D-8D-8D
 mode

On Tue, 5 May 2020 17:31:45 +0800
masonccyang@...c.com.tw wrote:


> > > > > I quickly went through your patches but can't reply them in each   
> your 
> > > > > patches.
> > > > > 
> > > > > i.e,.
> > > > > 1) [v4,03/16] spi: spi-mem: allow specifying a command's extension
> > > > > 
> > > > > -                                u8 opcode;
> > > > > +                                u16 opcode;
> > > > > 
> > > > > big/little Endian issue, right? 
> > > > > why not just u8 ext_opcode;
> > > > > No any impact for exist code and actually only xSPI device use   
> > > extension   
> > > > > command.  
> > > > 
> > > > Boris already explained the reasoning behind it.  
> > > 
> > > yup, I got his point and please make sure CPU data access.
> > > 
> > > i.e,.
> > > Fix endianness of the BFPT DWORDs and xSPI in sfdp.c
> > > 
> > > and your patch,
> > > +                                ext = spi_nor_get_cmd_ext(nor, op);
> > > +                                op->cmd.opcode = (op->cmd.opcode <<   
> 8) | 
> > > ext;
> > > +                                op->cmd.nbytes = 2;
> > > 
> > > I think maybe using u8 opcode[2] could avoid endianness.  
> > 
> > Again, thanks Boris for answering this. FWIW, I don't see anything wrong   
> 
> > with his suggestion.
> > 
> > To clarify a bit more, the idea is that we transmit the opcode MSB 
> > first, just we do for the address. Assume we want to issue the command 
> > 0x05. In 1S mode, we set cmd.opcode to 0x05. Here cmd.nbytes == 1. Treat   
> 
> > is as a 1-byte value, so the MSB is the same as the LSB. We directly 
> > send 0x5 on the bus.  
> 
> There are many SPI controllers driver use "op->cmd.opcode" directly,
> so is spi-mxic.c.
> 
> i.e,.
> ret = mxic_spi_data_xfer(mxic, &op->cmd.opcode, NULL, op->cmd.nbytes);

Just because you do it doesn't mean it's right. And most controllers use
the opcode value, they don't dereference the pointer as you do here.

> 
> > 
> > If cmd.nbytes == 2, then the opcode would be 0x05FA (assuming extension 
> > is invert of command). So we send the MSB (0x05) first, and LSB (0xFA) 
> > next.  
> 
> My platform is Xilinx Zynq platform which CPU is ARMv7 processor.
> 
> In 1-1-1 mode, it's OK to send 1 byte command by u16 opcode but
> in 8D-8D-8D mode, I need to patch
> 
> i.e.,
> op->cmd.opcode = op->cmd.opcode | (ext << 8);
> 
> rather than your patch.

Seriously, how hard is it to extract each byte from the u16 if your
controller needs to pass things in a different order? I mean, that's
already how it's done for the address cycle, so why is it a problem
here? This sounds like bikeshedding to me. If the order is properly
documented in the kernel doc, I see no problem having it grouped in one
u16, with the first cmd cycle placed in the MSB and the second one in
the LSB.