[PATCH v2] serial: mxc: Keep the original FIFO empty check

[Michael Nazzareno Trimarchi]

Hi Fabio

On Wed, Oct 26, 2022 at 2:18 AM Fabio Estevam <festevam at denx.de> wrote:
>
> Hi Tim,
>
> On 25/10/2022 20:23, Tim Harvey wrote:
>
> > Fabio,
> >
> > This resolves the issue. Why would the kernel not suffer from this as
> > well? We are essentially saying if the FIFO is not full but also not
> > empty we need to wait.... so do we have a FIFO size of 1 or something
> > here as opposed to the kernel?
>
> The FIFO has a fixed size of 32 chars.
>
> Kernel uses TXTL, which "controls the threshold at which a maskable
> interrupt is generated by the
> TxFIFO".
>
> We do not use interrupts in U-Boot. Prior to Johannes' patch, we only
> checked for the TXFIFO empty condition, which avoided sending data when
> nothing was
> available inside the FIFO (under-run condition).
>
> Johannes' patch handles over-run condition.
>
> My patch guarantees that both over-run and under-run conditions are
> handled.
>
> When the number of chars in the FIFO is 1, 2, 3,...30, the transmission
> can safely happen. Special cases are FIFO empty and FIFO full, which
> returns -EAGAIN to the serial core.

Can you explain it better? If FIFO is empty there are no chars in the
serial, and
the transmission can happen only when you have at least one byte in the fifo.
The underrun condition in general is when the start of tx is enabled
but you have no bytes to send
at the time that bytes are needed. Was the case to limit the change on
the architecture that was tested?

Michael

>
> Regards,
>
> Fabio Estevam
> --
> DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
> Phone: (+49)-8142-66989-60 Fax: (+49)-8142-66989-80 Email:
> festevam at denx.de



-- 
Michael Nazzareno Trimarchi
Co-Founder & Chief Executive Officer
M. +39 347 913 2170
michael at amarulasolutions.com
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