Re: [PATCH v2 2/3] spi: wpcm-fiu: Add driver for Nuvoton WPCM450 Flash Interface Unit (FIU)

["Jonathan Neuschäfer" <j.neuschaefer@gmx.net>]

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Hello,

On Fri, Nov 25, 2022 at 01:04:54PM +0000, Mark Brown wrote:
> On Thu, Nov 24, 2022 at 08:13:59PM +0100, Jonathan Neuschäfer wrote:
> 
> > The Flash Interface Unit (FIU) is the SPI flash controller in the
> > Nuvoton WPCM450 BMC SoC. It supports four chip selects, and direct
> > (memory-mapped) access to 16 MiB per chip. Larger flash chips can be
> > accessed by software-defined SPI transfers.
> 
> Those software defined SPI transfers seem to be most of the way to
> supporting normal SPI controller operations, they just need wiring up.
> That would both support people hooking other SPI chips up to the board
> and might help support future flash stuff without needing custom code in
> the driver like you've got now.

I'm not so sure. The hardware mechanism allowing "software defined" SPI
transfers is strongly oriented towards SPI flash, and it already felt
like a stretch to implement all the features that Linux expects of a
SPI MEM controller.

As to connecting non-memory chips: There is also a second, completely
different SPI controller in this SoC, which is used on some boards (in
factory configuration) to drive a little status LCD. I think it would be
easiest to use that one for custom hardware extensions.


> 
> > +static int wpcm_fiu_do_uma(struct wpcm_fiu_spi *fiu, unsigned int cs,
> > +			   bool use_addr, bool write, int data_bytes)
> > +{
> 
> This appears to only support half duplex access but the driver doesn't
> flag itself as SPI_CONTROLLER_HALF_DUPLEX.  

Ok, I'll add it.

> 
> > +	cts |= FIU_UMA_CTS_D_SIZE(data_bytes);
> 
> I'm guessing there's a limit on data_bytes that should be enforced.  The
> driver should probably also flag a max transfer size, though that might
> cause issues if the limit is different between spi-mem and regular
> transfers.

For the existing spi-mem case, the transfer size is limited through
wpcm_fiu_adjust_op_size. I *think* this is enough, but please correct me
if I'm wrong.


> > +/*
> > + * RDID (Read Identification) needs special handling because Linux expects to
> > + * be able to read 6 ID bytes and FIU can only read up to 4 at once.
> > + *
> > + * We're lucky in this case, because executing the RDID instruction twice will
> > + * result in the same result.
> > + *
> > + * What we do is as follows (C: write command/opcode byte, D: read data byte,
> > + * A: write address byte):
> > + *
> > + *  1. C D D D
> > + *  2. C A A A D D D
> > + */
> 
> If the driver were implementing regular SPI operations and advertising
> a maximum transfer length this should just work without having to jump
> through hoops.  The core can split transfers up into sections that fit
> within the controller limits transparently.

As far as I'm aware, the controller is not capable of performing a pure
read transfer, because the command byte (a byte that is written, in
half-duplex) is always included at the start. I think this limitation
would break your idea.

IOW, the hoops aren't nice, but I think they're necessary.


Thanks for your review,
Jonathan

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