Re: [PATCH v2] ns16550: enable memory decoding on MMIO-based PCI console card

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Apr 25, 2023 at 04:39:02PM +0200, Marek Marczykowski-Górecki wrote:
> pci_serial_early_init() enables PCI_COMMAND_IO for IO-based UART
> devices, add setting PCI_COMMAND_MEMORY for MMIO-based UART devices too.
> Note the MMIO-based devices in practice need a "pci" sub-option,
> otherwise a few parameters are not initialized (including bar_idx,
> reg_shift, reg_width etc). The "pci" is not supposed to be used with
> explicit BDF, so do not key setting PCI_COMMAND_MEMORY on explicit BDF
> being set. Contrary to the IO-based UART, pci_serial_early_init() will
> not attempt to set BAR0 address, even if user provided io_base manually
> - in most cases, those are with an offest and the current cmdline syntax
> doesn't allow expressing it. Due to this, enable PCI_COMMAND_MEMORY only
> if uart->bar is already populated. In similar spirit, this patch does
> not support setting BAR0 of the bridge.

FWIW (not that should be done here) but I think we also want to
disable memory decoding in pci_uart_config() while sizing the BAR.

> Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> ---
> This fixes the issue I was talking about on #xendevel. Thanks Roger for
> the hint.
> 
> Changes in v2:
>  - check if uart->bar instead of uart->io_base
>  - move it ahead of !uart->ps_bdf_enable return
>  - expand commit message.
> ---
>  xen/drivers/char/ns16550.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
> index 1b21eb93c45f..34231dcb23ea 100644
> --- a/xen/drivers/char/ns16550.c
> +++ b/xen/drivers/char/ns16550.c
> @@ -272,7 +272,15 @@ static int cf_check ns16550_getc(struct serial_port *port, char *pc)
>  static void pci_serial_early_init(struct ns16550 *uart)
>  {
>  #ifdef NS16550_PCI
> -    if ( !uart->ps_bdf_enable || uart->io_base >= 0x10000 )
> +    if ( uart->bar )
> +    {
> +        pci_conf_write16(PCI_SBDF(0, uart->ps_bdf[0], uart->ps_bdf[1],
> +                                  uart->ps_bdf[2]),
> +                         PCI_COMMAND, PCI_COMMAND_MEMORY);

Don't you want to read the current command register first and just
or PCI_COMMAND_MEMORY?

I see that for IO decoding we already do it this way, but I'm not sure
whether it could cause issues down the road by unintentionally
disabling command flags.

Thanks, Roger.