On 21/03/17 01:02PM, Leon Romanovsky wrote: > On Wed, Mar 17, 2021 at 03:54:47PM +0530, Amey Narkhede wrote: > > On 21/03/17 06:20AM, Leon Romanovsky wrote: > > > On Mon, Mar 15, 2021 at 06:32:32PM +0000, Raphael Norwitz wrote: > > > > On Mon, Mar 15, 2021 at 10:29:50AM -0600, Alex Williamson wrote: > > > > > On Mon, 15 Mar 2021 21:03:41 +0530 > > > > > Amey Narkhede <ameynarkhede03@xxxxxxxxx> wrote: > > > > > > > > > > > On 21/03/15 05:07PM, Leon Romanovsky wrote: > > > > > > > On Mon, Mar 15, 2021 at 08:34:09AM -0600, Alex Williamson wrote: > > > > > > > > On Mon, 15 Mar 2021 14:52:26 +0100 > > > > > > > > Pali Rohár <pali@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > On Monday 15 March 2021 19:13:23 Amey Narkhede wrote: > > > > > > > > > > slot reset (pci_dev_reset_slot_function) and secondary bus > > > > > > > > > > reset(pci_parent_bus_reset) which I think are hot reset and > > > > > > > > > > warm reset respectively. > > > > > > > > > > > > > > > > > > No. PCI secondary bus reset = PCIe Hot Reset. Slot reset is just another > > > > > > > > > type of reset, which is currently implemented only for PCIe hot plug > > > > > > > > > bridges and for PowerPC PowerNV platform and it just call PCI secondary > > > > > > > > > bus reset with some other hook. PCIe Warm Reset does not have API in > > > > > > > > > kernel and therefore drivers do not export this type of reset via any > > > > > > > > > kernel function (yet). > > > > > > > > > > > > > > > > Warm reset is beyond the scope of this series, but could be implemented > > > > > > > > in a compatible way to fit within the pci_reset_fn_methods[] array > > > > > > > > defined here. Note that with this series the resets available through > > > > > > > > pci_reset_function() and the per device reset attribute is sysfs remain > > > > > > > > exactly the same as they are currently. The bus and slot reset > > > > > > > > methods used here are limited to devices where only a single function is > > > > > > > > affected by the reset, therefore it is not like the patch you proposed > > > > > > > > which performed a reset irrespective of the downstream devices. This > > > > > > > > series only enables selection of the existing methods. Thanks, > > > > > > > > > > > > > > Alex, > > > > > > > > > > > > > > I asked the patch author here [1], but didn't get any response, maybe > > > > > > > you can answer me. What is the use case scenario for this functionality? > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > [1] https://lore.kernel.org/lkml/YE389lAqjJSeTolM@unreal/ > > > > > > > > > > > > > Sorry for not responding immediately. There were some buggy wifi cards > > > > > > which needed FLR explicitly not sure if that behavior is fixed in > > > > > > drivers. Also there is use a case at Nutanix but the engineer who > > > > > > is involved is on PTO that is why I did not respond immediately as > > > > > > I don't know the details yet. > > > > > > > > > > And more generally, devices continue to have reset issues and we > > > > > impose a fixed priority in our ordering. We can and probably should > > > > > continue to quirk devices when we find broken resets so that we have > > > > > the best default behavior, but it's currently not easy for an end user > > > > > to experiment, ie. this reset works, that one doesn't. We might also > > > > > have platform issues where a given reset works better on a certain > > > > > platform. Exposing a way to test these things might lead to better > > > > > quirks. In the case I think Pali was looking for, they wanted a > > > > > mechanism to force a bus reset, if this was in reference to a single > > > > > function device, this could be accomplished by setting a priority for > > > > > that mechanism, which would translate to not only the sysfs reset > > > > > attribute, but also the reset mechanism used by vfio-pci. Thanks, > > > > > > > > > > Alex > > > > > > > > > > > > > To confirm from our end - we have seen many such instances where default > > > > reset methods have not worked well on our platform. Debugging these > > > > issues is painful in practice, and this interface would make it far > > > > easier. > > > > > > > > Having an interface like this would also help us better communicate the > > > > issues we find with upstream. Allowing others to more easily test our > > > > (or other entities') findings should give better visibility into > > > > which issues apply to the device in general and which are platform > > > > specific. In disambiguating the former from the latter, we should be > > > > able to better quirk devices for everyone, and in the latter cases, this > > > > interface allows for a safer and more elegant solution than any of the > > > > current alternatives. > > > > > > So to summarize, we are talking about test and debug interface to > > > overcome HW bugs, am I right? > > > > > > My personal experience shows that once the easy workaround exists > > > (and write to generally available sysfs is very simple), the vendors > > > and users desire for proper fix decreases drastically. IMHO, we will > > > see increase of copy/paste in SO and blog posts, but reduce in quirks. > > > > > > My 2-cents. > > > > > I agree with your point but at least it gives the userspace ability > > to use broken device until bug is fixed in upstream. > > As I said, I don't expect many fixes once "userspace" will be able to > use cheap workaround. There is no incentive to fix it. > > > This is also applicable for obscure devices without upstream > > drivers for example custom FPGA based devices. > > This is not relevant to upstream kernel. Those vendors ship everything > custom, they don't need upstream, we don't need them :) > By custom I meant hobbyists who could tinker with their custom FPGA. > > Another main application which I forgot to mention is virtualization > > where vmm wants to reset the device when the guest is reset, > > to emulate machine reboot as closely as possible. > > It can work in very narrow case, because reset will cause to device > reprobe and most likely the driver will be different from the one that > started reset. I can imagine that net devices will lose their state and > config after such reset too. > Not sure if I got that 100% right. The pci_reset_function() function saves and restores device state over the reset. > IMHO, it will be saner for everyone if virtualization don't try such resets. > > Thanks > The exists reset sysfs attribute was added for exactly this case though. Thanks, Amey
