[U-Boot] [PATCH] dm: serial: Add .pre_probe() function to to wait for previous transmission end

[Lukasz Majewski]

Hi Philipp,

> Hi Lukasz,
> 
> > On 27 Oct 2017, at 00:51, Lukasz Majewski <lukma at denx.de> wrote:
> >> 
> >>> On 27 Oct 2017, at 00:13, Lukasz Majewski <lukma at denx.de> wrote:
> >>> 
> >>> It may happen that the serial IP block is performing some ongoing
> >>> transmission (started at e.g. board_init()) when the serial
> >>> "probe" is called.
> >>> 
> >>> As a result the serial port IP block is reset, so transmitted data
> >>> is corrupted:
> >>> 
> >>>   I2C:   ready
> >>>   DRAM:  1 GiB
> >>>   jSS('HH��SL_SDHC: 04 rev 0x0
> >>> 
> >>> This patch prevents from this situation, by defining pre_probe()
> >>> callback in which we wait till the TX buffer is empty (from
> >>> previous transmission):
> >>> 
> >>>   I2C:   ready
> >>>   DRAM:  1 GiB
> >>>   ID:    unit type 0x4 rev 0x0
> >>> 
> >>> All defined ->pending callbacks at ./drivers/serial are non
> >>> blocking
> >>> - just simple reading from registers and testing flags. Hence, it
> >>> should be enough to not use any timeout from timer.
> >>> One shall also note that we enable console very early - not all
> >>> timers may be ready for work - adding timeout here would impose
> >>> implicit dependency that timers are setup before serial.
> >> 
> >> Given that this is effectively a busy polling loop, why can’t this
> >> be done from the probe-function of serial drivers that require this
> >> functionality?
> > 
> > That would be one of the options. 
> > 
> > Originally, this code was placed at iMX specific function - namely
> > _mxc_serial_init() [1]. 
> > 
> > However, Simon suggested to solve this problem globally via DM's
> > serial-uclass, which offers pre_probe() callback for such purpose.
> > 
> > The problem here is the polling loop. I've checked and ->pending on
> > real SoCs is just a read from the register - which should not block
> > (a similar approach is used in Linux kernel).
> > 
> > Having timeout from timer would impose dependency on timer init'ed
> > first - before serial.
> 
> I worry a bit about the pending-callback being called prior to a
> probe. As of today, the pending function can assume that probe() has
> run and initialised the private structures accordingly—with this
> change, that assumption is invalidated.

Yes. I agree.

Other issue is that we may need timer based timeout for some SoCs. And
this also implies to have timer initialized before the console.

> 
> If we go down this path, then we need to clearly indicate that the
> pending function can not rely on probe() to have initialised the
> device state or private data structures.  If we keep this check
> within the probe()-function, it should be obvious what is set up and
> what isn’t.

Those are valid arguments.

I would even go further and leave this patch as it was in the first
version [1] - since:

- It is similar to what Linux does
- for iMX it uses check on HW bit which is guarantee to be cleared in
  some point of time (of course if HW is not broken).

However, lets wait for input from Simon - how he would like to tackle
this issue.

> 
> > 
> > [1] - https://patchwork.ozlabs.org/patch/820824/
> > 
> >> 
> >>> 
> >>> 
> >>> Signed-off-by: Lukasz Majewski <lukma at denx.de>
> >>> ---
> >>> 
> >>> drivers/serial/serial-uclass.c | 20 ++++++++++++++++++++
> >>> 1 file changed, 20 insertions(+)
> >>> 
> >>> diff --git a/drivers/serial/serial-uclass.c
> >>> b/drivers/serial/serial-uclass.c index 2e5116f..5e6964d 100644
> >>> --- a/drivers/serial/serial-uclass.c
> >>> +++ b/drivers/serial/serial-uclass.c
> >>> @@ -420,10 +420,30 @@ static int serial_pre_remove(struct udevice
> >>> *dev) return 0;
> >>> }
> >>> 
> >>> +static int serial_pre_probe(struct udevice *dev)
> >>> +{
> >>> +	struct dm_serial_ops *ops = serial_get_ops(dev);
> >>> +	int ret = 0;
> >>> +
> >>> +	/*
> >>> +	 * Wait for any ongoing transmission to finish - for
> >>> example
> >>> +	 * from pre-relocation enabled UART
> >>> +	 */
> >>> +	if (ops && ops->pending)
> >>> +		do {
> >>> +			ret = ops->pending(dev, false);
> >>> +			if (ret < 0)
> >>> +				break;
> >>> +		} while (ret > 0);
> >>> +
> >>> +	return ret;
> >>> +}
> >>> +
> >>> UCLASS_DRIVER(serial) = {
> >>> 	.id		= UCLASS_SERIAL,
> >>> 	.name		= "serial",
> >>> 	.flags		= DM_UC_FLAG_SEQ_ALIAS,
> >>> +	.pre_probe	= serial_pre_probe,
> >>> 	.post_probe	= serial_post_probe,
> >>> 	.pre_remove	= serial_pre_remove,
> >>> 	.per_device_auto_alloc_size = sizeof(struct
> >>> serial_dev_priv), -- 
> >>> 2.1.4
> >>> 
> >> 
> > 
> > 
> > 
> > 
> > Best regards,
> > 
> > Lukasz Majewski
> > 
> > --
> > 
> > DENX Software Engineering GmbH,      Managing Director: Wolfgang
> > Denk HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell,
> > Germany Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email:
> > wd at denx.de
> 



Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de