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SubjectRE: [PATCH 03/14] serial: tegra: avoid reg access when clk disabled
Date
> -----Original Message-----
> From: linux-tegra-owner@vger.kernel.org <linux-tegra-
> owner@vger.kernel.org> On Behalf Of Thierry Reding
> Sent: Tuesday, August 13, 2019 3:16 PM
> To: Krishna Yarlagadda <kyarlagadda@nvidia.com>
> Cc: gregkh@linuxfoundation.org; robh+dt@kernel.org;
> mark.rutland@arm.com; Jonathan Hunter <jonathanh@nvidia.com>; Laxman
> Dewangan <ldewangan@nvidia.com>; jslaby@suse.com; linux-
> serial@vger.kernel.org; devicetree@vger.kernel.org; linux-
> tegra@vger.kernel.org; linux-kernel@vger.kernel.org; Ahung Cheng
> <ahcheng@nvidia.com>; Shardar Mohammed <smohammed@nvidia.com>
> Subject: Re: [PATCH 03/14] serial: tegra: avoid reg access when clk
> disabled
>
> On Mon, Aug 12, 2019 at 04:58:12PM +0530, Krishna Yarlagadda wrote:
> > From: Ahung Cheng <ahcheng@nvidia.com>
> >
> > This avoids two race conditions from the UART shutdown sequence both
> > leading to 'Machine check error in AXI2APB' and kernel oops.
> >
> > One was that the clock was disabled before the DMA was terminated
> > making it possible for the DMA callbacks to be called after the
> > clock was disabled. These callbacks could write to the UART
> > registers causing timeout.
> >
> > The second was that the clock was disabled before the UART was
> > completely flagged as closed. This is done after the shutdown is
> > called and a new write could be started after the clock was disabled.
> > tegra_uart_start_pio_tx could be called causing timeout.
> >
> > Given that the baud rate is reset at the end of shutdown sequence,
> > this fix is to examine the baud rate to avoid register access from
> > both race conditions.
> >
> > Besides, terminate the DMA before disabling the clock.
> >
> > Signed-off-by: Ahung Cheng <ahcheng@nvidia.com>
> > Signed-off-by: Shardar Mohammed <smohammed@nvidia.com>
> > Signed-off-by: Krishna Yarlagadda <kyarlagadda@nvidia.com>
> > ---
> > drivers/tty/serial/serial-tegra.c | 17 +++++++++++------
> > 1 file changed, 11 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/tty/serial/serial-tegra.c
> > b/drivers/tty/serial/serial-tegra.c
> > index 93d299e..d908465 100644
> > --- a/drivers/tty/serial/serial-tegra.c
> > +++ b/drivers/tty/serial/serial-tegra.c
> > @@ -126,6 +126,8 @@ struct tegra_uart_port {
> >
> > static void tegra_uart_start_next_tx(struct tegra_uart_port *tup);
> > static int tegra_uart_start_rx_dma(struct tegra_uart_port *tup);
> > +static void tegra_uart_dma_channel_free(struct tegra_uart_port *tup,
> > + bool dma_to_memory);
> >
> > static inline unsigned long tegra_uart_read(struct tegra_uart_port *tup,
> > unsigned long reg)
> > @@ -458,6 +460,9 @@ static void tegra_uart_start_next_tx(struct
> tegra_uart_port *tup)
> > unsigned long count;
> > struct circ_buf *xmit = &tup->uport.state->xmit;
> >
> > + if (WARN_ON(!tup->current_baud))
> > + return;
>
> Are the race conditions that you are describing something which can be
> triggered by the user? If so, it's not a good idea to use a WARN_ON,
> because that could lead to some userspace spamming the log with these,
> potentially on purpose.
>
> Thierry
>
These are triggered by user events. I will remove WARN_ON

KY
> > +
> > tail = (unsigned long)&xmit->buf[xmit->tail];
> > count = CIRC_CNT_TO_END(xmit->head, xmit->tail,
> UART_XMIT_SIZE);
> > if (!count)
> > @@ -829,6 +834,12 @@ static void tegra_uart_hw_deinit(struct
> tegra_uart_port *tup)
> > tup->current_baud = 0;
> > spin_unlock_irqrestore(&tup->uport.lock, flags);
> >
> > + tup->rx_in_progress = 0;
> > + tup->tx_in_progress = 0;
> > +
> > + tegra_uart_dma_channel_free(tup, true);
> > + tegra_uart_dma_channel_free(tup, false);
> > +
> > clk_disable_unprepare(tup->uart_clk);
> > }
> >
> > @@ -1066,12 +1077,6 @@ static void tegra_uart_shutdown(struct
> uart_port *u)
> > struct tegra_uart_port *tup = to_tegra_uport(u);
> >
> > tegra_uart_hw_deinit(tup);
> > -
> > - tup->rx_in_progress = 0;
> > - tup->tx_in_progress = 0;
> > -
> > - tegra_uart_dma_channel_free(tup, true);
> > - tegra_uart_dma_channel_free(tup, false);
> > free_irq(u->irq, tup);
> > }
> >
> > --
> > 2.7.4
> >

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