linux-kernel - Re: [PATCH v3 2/8] serial: sh-sci: Check if TX data was written to device in .tx

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Message-ID: <bf8aa2ca-8a5e-4484-8f93-c74b7c6e0db9@tuxon.dev>
Date: Fri, 22 Nov 2024 10:17:46 +0200
From: Claudiu Beznea <claudiu.beznea@...on.dev>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: geert+renesas@...der.be, magnus.damm@...il.com, robh@...nel.org,
 krzk+dt@...nel.org, conor+dt@...nel.org, mturquette@...libre.com,
 sboyd@...nel.org, jirislaby@...nel.org, p.zabel@...gutronix.de,
 lethal@...ux-sh.org, g.liakhovetski@....de,
 linux-renesas-soc@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
 linux-serial@...r.kernel.org,
 Claudiu Beznea <claudiu.beznea.uj@...renesas.com>, stable@...r.kernel.org
Subject: Re: [PATCH v3 2/8] serial: sh-sci: Check if TX data was written to
 device in .tx_empty()

Hi, Greg,

On 21.11.2024 23:32, Greg KH wrote:
> On Fri, Nov 15, 2024 at 03:43:55PM +0200, Claudiu wrote:
>> From: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
>>
>> On the Renesas RZ/G3S, when doing suspend to RAM, the uart_suspend_port()
>> is called. The uart_suspend_port() calls 3 times the
>> struct uart_port::ops::tx_empty() before shutting down the port.
>>
>> According to the documentation, the struct uart_port::ops::tx_empty()
>> API tests whether the transmitter FIFO and shifter for the port is
>> empty.
>>
>> The Renesas RZ/G3S SCIFA IP reports the number of data units stored in the
>> transmit FIFO through the FDR (FIFO Data Count Register). The data units
>> in the FIFOs are written in the shift register and transmitted from there.
>> The TEND bit in the Serial Status Register reports if the data was
>> transmitted from the shift register.
>>
>> In the previous code, in the tx_empty() API implemented by the sh-sci
>> driver, it is considered that the TX is empty if the hardware reports the
>> TEND bit set and the number of data units in the FIFO is zero.
>>
>> According to the HW manual, the TEND bit has the following meaning:
>>
>> 0: Transmission is in the waiting state or in progress.
>> 1: Transmission is completed.
>>
>> It has been noticed that when opening the serial device w/o using it and
>> then switch to a power saving mode, the tx_empty() call in the
>> uart_port_suspend() function fails, leading to the "Unable to drain
>> transmitter" message being printed on the console. This is because the
>> TEND=0 if nothing has been transmitted and the FIFOs are empty. As the
>> TEND=0 has double meaning (waiting state, in progress) we can't
>> determined the scenario described above.
>>
>> Add a software workaround for this. This sets a variable if any data has
>> been sent on the serial console (when using PIO) or if the DMA callback has
>> been called (meaning something has been transmitted). In the tx_empty()
>> API the status of the DMA transaction is also checked and if it is
>> completed or in progress the code falls back in checking the hardware
>> registers instead of relying on the software variable.
>>
>> Fixes: 73a19e4c0301 ("serial: sh-sci: Add DMA support.")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
> 
> Why is this bug/regression fix burried in a long series?  It should be
> sent individually so that it could be applied on its own as it is not
> related to the other ones, right?

It is related to the suspend to RAM support added in this series.

> 
> Or are you ok with waiting for this to show up in 6.14-rc1?

I'll resend it individually.

Thank you,
Claudiu Beznea

> 
> thanks,
> 
> greg k-h