[PATCH v3] drivers/tty: amba: defer probing DMA availability until hw_init

[Rob Herring]

On Wed, Feb 25, 2015 at 6:31 PM, Jorge Ramirez-Ortiz
<jorge.ramirez-ortiz at linaro.org> wrote:
> Fix a race condition that happens when device_initcall(pl011_dma_initicall)
> is executed before all the devices have been probed - this issue was observed on
> an hisi_6220 SoC (HiKey board from Linaro).
>
> The deferred driver probing framework relies on late_initcall to trigger
> deferred probes so it is just possible that, even with a valid DMA driver ready
> to be loaded, we fail to synchronize with it.
>
> The proposed implementation delays probing of the DMA until the uart device is
> opened. As hw_init is invoked on port startup and port resume - but the DMA
> probe is only required once - we avoid calling multiple times using a new field
> in uart_amba_port to track this scenario.
>
> Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz at linaro.org>
> ---
>  drivers/tty/serial/amba-pl011.c | 66 ++++++++++-------------------------------
>  1 file changed, 16 insertions(+), 50 deletions(-)
>
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index 8d94c19..3501ccc 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
> @@ -29,6 +29,7 @@
>   * and hooked into this driver.
>   */
>
> +#define pr_fmt(fmt) "amba-pl011: "fmt

You can drop this I think.

>  #if defined(CONFIG_SERIAL_AMBA_PL011_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
>  #define SUPPORT_SYSRQ
> @@ -164,6 +165,7 @@ struct uart_amba_port {
>         bool                    using_rx_dma;
>         struct pl011_dmarx_data dmarx;
>         struct pl011_dmatx_data dmatx;
> +       bool                    dma_probed;
>  #endif
>  };
>
> @@ -261,7 +263,7 @@ static void pl011_sgbuf_free(struct dma_chan *chan, struct pl011_sgbuf *sg,
>         }
>  }
>
> -static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *uap)
> +static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
>  {
>         /* DMA is the sole user of the platform data right now */
>         struct amba_pl011_data *plat = dev_get_platdata(uap->port.dev);
> @@ -275,9 +277,16 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>         struct dma_chan *chan;
>         dma_cap_mask_t mask;
>
> -       chan = dma_request_slave_channel(dev, "tx");
> +       uap->dma_probed = true;

I would not set this to true in the case of EPROBE_DEFER. That way if
a DMA driver is loaded as a module some time later, DMA can be used.
We're not really using deferred probe here, but using it as an
indication that the UART is DMA capable.

> +
> +       chan = dma_request_slave_channel_reason(dev, "tx");
> +       if (IS_ERR(chan)) {
> +               if (PTR_ERR(chan) == -EPROBE_DEFER) {
> +                       dev_warn(uap->port.dev, "DMA driver not ready\n");
> +                       return;
> +               }
> +               dev_info(uap->port.dev, "no OF or ACPI DMA data\n");
>
> -       if (!chan) {
>                 /* We need platform data */
>                 if (!plat || !plat->dma_filter) {
>                         dev_info(uap->port.dev, "no DMA platform data\n");
> @@ -383,54 +392,9 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>                 dev_info(uap->port.dev, "DMA channel RX %s\n",
>                          dma_chan_name(uap->dmarx.chan));
>         }
> -}
> -
> -#ifndef MODULE
> -/*
> - * Stack up the UARTs and let the above initcall be done at device
> - * initcall time, because the serial driver is called as an arch
> - * initcall, and at this time the DMA subsystem is not yet registered.
> - * At this point the driver will switch over to using DMA where desired.
> - */
> -struct dma_uap {
> -       struct list_head node;
> -       struct uart_amba_port *uap;
> -       struct device *dev;
> -};
> -
> -static LIST_HEAD(pl011_dma_uarts);
> -
> -static int __init pl011_dma_initcall(void)
> -{
> -       struct list_head *node, *tmp;
> -
> -       list_for_each_safe(node, tmp, &pl011_dma_uarts) {
> -               struct dma_uap *dmau = list_entry(node, struct dma_uap, node);
> -               pl011_dma_probe_initcall(dmau->dev, dmau->uap);
> -               list_del(node);
> -               kfree(dmau);
> -       }
> -       return 0;
> -}
> -
> -device_initcall(pl011_dma_initcall);
>
> -static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
> -{
> -       struct dma_uap *dmau = kzalloc(sizeof(struct dma_uap), GFP_KERNEL);
> -       if (dmau) {
> -               dmau->uap = uap;
> -               dmau->dev = dev;
> -               list_add_tail(&dmau->node, &pl011_dma_uarts);
> -       }
> +       return;

You don't need an explicit return.

>  }
> -#else
> -static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
> -{
> -       pl011_dma_probe_initcall(dev, uap);
> -}
> -#endif
> -
>  static void pl011_dma_remove(struct uart_amba_port *uap)
>  {
>         /* TODO: remove the initcall if it has not yet executed */
> @@ -1548,6 +1512,9 @@ static int pl011_hwinit(struct uart_port *port)
>         uap->im = readw(uap->port.membase + UART011_IMSC);
>         writew(UART011_RTIM | UART011_RXIM, uap->port.membase + UART011_IMSC);
>
> +       if (!uap->dma_probed)
> +               pl011_dma_probe(uap->port.dev, uap);
> +
>         if (dev_get_platdata(uap->port.dev)) {
>                 struct amba_pl011_data *plat;
>
> @@ -2218,7 +2185,6 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
>         uap->port.ops = &amba_pl011_pops;
>         uap->port.flags = UPF_BOOT_AUTOCONF;
>         uap->port.line = i;
> -       pl011_dma_probe(&dev->dev, uap);
>
>         /* Ensure interrupts from this UART are masked and cleared */
>         writew(0, uap->port.membase + UART011_IMSC);
> --
> 1.9.1
>