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

[Bin Meng]

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.

Regards,
Bin