Re: [PATCH v2 8/8] xen/arm: Add support for SMMUv3 driver

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 3 Dec 2020, Rahul Singh wrote:
> > On 3 Dec 2020, at 4:13 am, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> > wrote:
> > On Wed, 2 Dec 2020, Julien Grall wrote:
> >> On 02/12/2020 02:51, Stefano Stabellini wrote:
> >>> On Thu, 26 Nov 2020, Rahul Singh wrote:
> >>>> +/* Alias to Xen device tree helpers */
> >>>> +#define device_node dt_device_node
> >>>> +#define of_phandle_args dt_phandle_args
> >>>> +#define of_device_id dt_device_match
> >>>> +#define of_match_node dt_match_node
> >>>> +#define of_property_read_u32(np, pname, out) (!dt_property_read_u32(np,
> >>>> pname, out))
> >>>> +#define of_property_read_bool dt_property_read_bool
> >>>> +#define of_parse_phandle_with_args dt_parse_phandle_with_args
> >>> 
> >>> Given all the changes to the file by the previous patches we are
> >>> basically fully (or almost fully) adapting this code to Xen.
> >>> 
> >>> So at that point I wonder if we should just as well make these changes
> >>> (e.g. s/of_phandle_args/dt_phandle_args/g) to the code too.
> >> 
> >> I have already accepted the fact that keeping Linux code as-is is nearly
> >> impossible without much workaround :). The benefits tends to also limited 
> >> as
> >> we noticed for the SMMU driver.
> >> 
> >> I would like to point out that this may make quite difficult (if not
> >> impossible) to revert the previous patches which remove support for some
> >> features (e.g. atomic, MSI, ATS).
> >> 
> >> If we are going to adapt the code to Xen (I'd like to keep Linux code style
> >> though), then I think we should consider to keep code that may be useful in
> >> the near future (at least MSI, ATS).
> > 
> > (I am fine with keeping the Linux code style.)
> > 
> > We could try to keep the code as similar to Linux as possible. This
> > didn't work out in the past.
> > 
> > Otherwise, we could fully adapt the driver to Xen. If we fully adapt the
> > driver to Xen (code style aside) it is better to be consistent and also
> > do substitutions like s/of_phandle_args/dt_phandle_args/g. Then the
> > policy becomes clear: the code comes from Linux but it is 100% adapted
> > to Xen.
> > 
> > 
> > Now the question about what to do about the MSI and ATS code is a good
> > one. We know that we are going to want that code at some point in the
> > next 2 years. Like you wrote, if we fully adapt the code to Xen and
> > remove MSI and ATS code, then it is going to be harder to add it back.
> > 
> > So maybe keeping the MSI and ATS code for now, even if it cannot work,
> > would be better. I think this strategy works well if the MSI and ATS
> > code can be disabled easily, i.e. with a couple of lines of code in the
> > init function rather than #ifdef everywhere. It doesn't work well if we
> > have to add #ifdef everywhere.
> > 
> > It looks like MSI could be disabled adding a couple of lines to
> > arm_smmu_setup_msis.
> > 
> > Similarly ATS seems to be easy to disable by forcing ats_enabled to
> > false.
> > 
> > So yes, this looks like a good way forward. Rahul, what do you think?
> 
> 
> I am ok to have the PCI ATS and MSI functionality in the code. 
> As per the discussion next version of the patch will include below 
> modification:Please let me know if there are any suggestion overall that 
> should be added in next version.
> 
> 1. Keep the PCI ATS and MSI functionality code.

I'll repeat one point I wrote above that I think it is important: please
try to disable ATS and MSI without invasive changes, ideally just a
couple of lines to force-disable each feature.


> 2. Make the code with XEN compatible ( remove linux compatibility functions)
> 3. Keep the Linux coding style for code imported from Linux.
> 4. Fix all comments.

Sounds good.


> I have one query what will be coding style for new code to make driver work  
> for XEN ? 

We try to keep the same code style for the entirety of a source file. In
this case, the whole driver should use Linux code style (both imported
code and new code).