Re: [PATCH] hw/ssi: imx_spi: Improve chip select handling

[Guenter Roeck]

On Sun, Sep 26, 2021 at 10:49:53AM +0800, Bin Meng wrote:
> On Sat, Sep 18, 2021 at 12:19 PM Guenter Roeck <linux@roeck-us.net> wrote:
> >
> > On 9/17/21 8:09 PM, Cheng, Xuzhou wrote:
> > >>> I got some free time in the past days to investigate this issue. 
> > >>> Guenter is right, the Linux imx-spi driver does not work on QEMU.
> > >>>
> > >>> The reason is that the state of m25p80 machine loops in 
> > >>> STATE_READING_DATA state after receiving RDSR command, the new command 
> > >>> is ignored. Before sending a new command, the CS line should be pulled 
> > >>> to high, this make the state of m25p80 back to IDLE.
> > >>>
> > >>> I have same point with Guenter, it's that set CS to 1 when burst is 
> > >>> zero. But i don't think it is necessary to set CS to 0 in 
> > >>> imx_spi_flush_txfifo(). I will send a new patch to fix this issue.
> > >>>
> > >>
> > >> Thanks a lot for looking into this. If you have a better solution than 
> > >> mine, by all means, please go for it. As I mentioned in my patch, I 
> > >> didn't really like it, but I was unable to find a better solution.
> > > I am doing some experiment to verify that the new patch is reasonable, so 
> > > the new patch will be delayed few days.
> > >
> >
> > No problem. Note that I'll be traveling for the next 2-3 weeks, and I won't 
> > be able
> > to test any patches during that time.
> >
> 
> I have some updates to share, as I have been working with Xuzhou
> internally on this issue for the past days:
> 
> Current mods using BURST_LEN to determine the timing to pull up the CS
> line in the SPI controller codes is a workaround. Hardware does not do
> this. To understand what real hardware behavior is, Xuzhou used an
> oscilloscope to verify our guess.
> 
> It turns out the root cause is elsewhere, and a proper fix will be
> sent out soon.
> 

Thanks a lot for tracking this down!

Guenter