On 7/11/19 1:07 AM, Ralf Ramsauer wrote:
> On 7/10/19 7:22 PM, Jan Kiszka wrote:
>> On 10.07.19 18:28, Ralf Ramsauer wrote:
>>>
>>> On 7/10/19 6:13 PM, Jan Kiszka wrote:
>>>>> Ok, there's one chance left: According to lspci, the device actually
>>>>> supports MSI. It's just not being used by Linux. Maybe I can somehow
>>>>> convince Linux to switch to MSI.
>>>> Latest kernel already?
>
> Latest kernel doesn't change anything.
>
>>>>
>>>
>>> I'm using the 4.19-rt jailhouse. Do you know of any relevant changes
>>> upstream? Anyway, it's worth a try, let me test upstream...
>>>
>>> At least for 4.19, it looks like I'm not able to easily switch to MSI.
>>> At the moment, I don't even understand why the kernel actually uses
>>> legacy interrupts, while MSI cap is present. Something is odd there, I'd
>>> expect that MSI is, if available, the perefered method. But even the
>>> root-cell after boot won't enable MSI. Still investigating...
>>>
>>> I hope that I can switch to MSI with maybe a dirty hack, but I need more
>>> time to dig through the kernel's PCI stack (PCI starts bugging me). You
>>> can find things there you never wanted to know! :-)
>>
>> I suspect you are using some 8250 derivative with a similar driver:
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=172c33cb61da0df5ccbdf1a8e736c8837d165a00
>>
>> Check your concrete driver for its interrupt allocation.
>
> Thanks for the pointer, this patch is really helpful and almost exactly
> what I'm looking for. I want to test the required changes of the driver
> on Linux without Jailhouse first, before I'm going to test it in a
> non-root cell.
>
> My device uses the standard 8250_pci driver. So I hooked in there, and
> tried the following hack:
>
>
> diff --git a/drivers/tty/serial/8250/8250_pci.c
> b/drivers/tty/serial/8250/8250_pci.c
> index bbe5cba21522..55f8144ef3e0 100644
> --- a/drivers/tty/serial/8250/8250_pci.c
> +++ b/drivers/tty/serial/8250/8250_pci.c
> @@ -3640,10 +3640,20 @@ pciserial_init_ports(struct pci_dev *dev, const
> struct pciserial_board *board)
> priv->dev = dev;
> priv->quirk = quirk;
>
> + pci_set_master(dev);
> +
> + rc = pci_alloc_irq_vectors(dev, 1, 1, PCI_IRQ_ALL_TYPES);
> + if (rc < 0) {
> + kfree(priv);
> + priv = ERR_PTR(rc);
> + goto err_deinit;
> + }
> +
> memset(&uart, 0, sizeof(uart));
> uart.port.flags = UPF_SKIP_TEST | UPF_BOOT_AUTOCONF | UPF_SHARE_IRQ;
> uart.port.uartclk = board->base_baud * 16;
> - uart.port.irq = get_pci_irq(dev, board);
> + uart.port.irq = pci_irq_vector(dev, 0);
> uart.port.dev = &dev->dev;
>
> for (i = 0; i < nr_ports; i++) {
>
>
> Luckily, probing succeeds. For the device, lspci reports:
>
> 01:00.0 Serial controller: Device 1c00:3253 (rev 10) (prog-if 05 [16850])
> [...]
> Capabilities: [68] MSI: Enable+ Count=1/32 Maskable+ 64bit+
> Address: 00000000fee00438 Data: 0000
> Masking: ffffffff Pending: 00000000
>
> Doesn't look bad so far. MSI is enabled and in use. And as soon as I
> write to or read from the device (echo / cat), the MSI interrupt appears
> in /proc/interrupts.
>
> But cat won't receive anything, and echo won't write anything. In fact,
> echoing to the device stalls until I abort with ^C. In both cases, no
> interrupts occur (according to /proc/interrupts).
>
> That would have been too easy.
Turns out it is that easy. Code above works perfect, but the serial
card's MSI support seems to be broken. So I went for another PCIe serial
card, and now it works fine.
Let me just advertise this product, in case someone else runs into the
same issue. [1] comes with two serial lines -- one PCI function per
line, which is pretty nice. Didn't try it yet, but it should be possible
to assign ports to different cells. Yet, the kernel needs to be patched
in order to enable MSI.
Ralf
[1] https://www.amazon.de/gp/product/B0073LM0S4
>
> Thanks
> Ralf
>
>>
>> Jan
>>
>
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