Re: [PATCH 5.10.y 1/4] serial: sh-sci: Check if TX data was written to device in .tx_empty()

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jun 03, 2025 at 12:36:58PM +0300, Claudiu wrote:
> From: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
> 
> commit 7cc0e0a43a91052477c2921f924a37d9c3891f0c upstream.
> 
> 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@vger.kernel.org
> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
> Link: https://lore.kernel.org/r/20241125115856.513642-1-claudiu.beznea.uj@bp.renesas.com
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> [claudiu.beznea: fixed conflict by:
>  - keeping serial_port_out() instead of sci_port_out() in
>    sci_transmit_chars()
>  - keeping !uart_circ_empty(xmit) condition in sci_dma_tx_complete(),
>    after s->tx_occurred = true; assignement]
> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
> ---
>  drivers/tty/serial/sh-sci.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)


For obvious reasons, we can't take patches ONLY for older stable
kernels, and not newer ones.  Otherwise you would have kernel releases
without any fixes and get a regression when moving to a newer tree,
which you yourself don't want to have happen :)

So can you resend these patches for all of the needed branches,
including this one, as I'll drop this from my review queue now.

thanks,

greg k-h