[PATCH] lib: utils/serial: Modify uart_getc to a blocking function

[Xiang W]

在 2021-11-14星期日的 00:31 +0000，Jessica Clarke写道：
> On 14 Nov 2021, at 00:23, Xiang W <wxjstz at 126.com> wrote:
> > 在 2021-11-13星期六的 22:11 +0000，Jessica Clarke写道：
> > > On 13 Nov 2021, at 07:06, Xiang W <wxjstz at 126.com> wrote:
> > > > 
> > > > If uart_getc is non-blocking, if the sending end is faster than
> > > > the
> > > > receiving end, the data received by some functions(sbi_gets) is
> > > > incomplete
> > > 
> > > Without hardware flow control this is always going to be true.
> > > Making
> > > getc blocking does not change that. Moreover, making it blocking
> > > means
> > > there is no longer a way to poll, whereas having it be non-
> > > blocking
> > > means you can poll, and if you want to make it blocking you can
> > > always
> > > put a loop in the caller (which also avoids putting it in every
> > > single
> > > driver).
> > > 
> > > Also, this breaks the SBI getchar call; it’s supposed to be
> > > non-blocking, but this makes it blocking, so it is impossible for
> > > an
> > > OS
> > > to poll the SBI console.
> > 
> > uart get does not support and returns the same value without
> > receiving
> > data. So polling will also cause problems
> > 
> > int sbi_getc(void)
> > {
> >         if (console_dev && console_dev->console_getc)
> >                 // Return -1 without valid data
> >                 return console_dev->console_getc();
> >         return -1;
Hi, Jess

Can we return SBI_ENOTSUPP(-2) here to identify that
console_dev->console_getc is not implemented.
> > }
> 
> I do not understand what you are saying. There is no problem here. If
> there is no character available, either because there is no UART or
> because the UART’s receive buffer has been drained, the OS is told
> there is no character. What’s wrong with that?
> 
> The only problem I can see is that BBL returns 0 when there is no
> UART
> and -1 when there’s a UART but no character available, so this is not
> a
> correct implementation of the legacy SBI, but probably a more
> sensible
> interface given 0 is a valid character.
I think 0 is not a good idea. The value obtained by getc is 0-255,
which includes 0.

Regards,
Xiang W
> Jess
> 
>