linux-kernel - Re: [PATCH] serial: 8250_lpss: Unconditionally set PCI master for Quark

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Date:   Mon, 9 Jan 2017 18:00:16 +0100
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] serial: 8250_lpss: Unconditionally set PCI master for
 Quark

On 2017-01-05 22:54, Andy Shevchenko wrote:
> On Wed, Jan 4, 2017 at 10:48 PM, Jan Kiszka <jan.kiszka@...mens.com> wrote:
>> MSI needs it as well.
>>
>> Should have no practical impact, though, as DMA is always available on
>> the Quark. But given the few users of pci_alloc_irq_vectors so far, this
>> incorrect pattern may spread otherwise.
>>
> 
> One question below.
> 
> Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>
> 
>> Fixes: 3f3a46951e02 ("serial: 8250_lpss: set PCI master only for private DMA")
>> Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
>> ---
>>  drivers/tty/serial/8250/8250_lpss.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/serial/8250/8250_lpss.c b/drivers/tty/serial/8250/8250_lpss.c
>> index f09f68a..9315197 100644
>> --- a/drivers/tty/serial/8250/8250_lpss.c
>> +++ b/drivers/tty/serial/8250/8250_lpss.c
>> @@ -183,7 +183,6 @@ static void qrk_serial_setup_dma(struct lpss8250 *lpss, struct uart_port *port)
>>         if (ret)
>>                 return;
>>
>> -       pci_set_master(pdev);
>>         pci_try_set_mwi(pdev);
> 
> Does it make sense to move MWI there as well?

TBH, I didn't come across the need to enable this bit so far,
specifically not for doing MSI transactions. Is MWI used at all for MSI
(spec says that MSI is "using a PCI DWORD memory write transaction")?
That question is better answered by someone more familiar with such PCI
details.

Jan

> 
>>
>>         /* Special DMA address for UART */
>> @@ -216,6 +215,8 @@ static int qrk_serial_setup(struct lpss8250 *lpss, struct uart_port *port)
>>         struct pci_dev *pdev = to_pci_dev(port->dev);
>>         int ret;
>>
>> +       pci_set_master(pdev);
>> +
>>         ret = pci_alloc_irq_vectors(pdev, 1, 1, PCI_IRQ_ALL_TYPES);
>>         if (ret < 0)
>>                 return ret;
>> --
>> 2.1.4
> 
> 
> 

-- 
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