Re: [PATCH 2/2] firmware: arm_scmi: Add qcom hvc/shmem transport

[Nikunj Kela]

On 7/18/2023 12:07 PM, Bjorn Andersson wrote:
> On Tue, Jul 18, 2023 at 11:53:24AM -0700, Nikunj Kela wrote:
> > On 7/18/2023 11:29 AM, Bjorn Andersson wrote:
> > > On Tue, Jul 18, 2023 at 09:08:33AM -0700, Nikunj Kela wrote:
> > > > diff --git a/drivers/firmware/arm_scmi/qcom_hvc.c b/drivers/firmware/arm_scmi/qcom_hvc.c
> [..]
> > > > +static int qcom_hvc_chan_setup(struct scmi_chan_info *cinfo,
> > > > +			       struct device *dev, bool tx)
> > > > +{
> [..]
> > > > +	/* let's map 2 additional ulong since
> > > > +	 * func-id & capability-id are kept after shmem.
> > > > +	 *     +-------+
> > > > +	 *     |       |
> > > > +	 *     | shmem |
> > > > +	 *     |       |
> > > > +	 *     |       |
> > > > +	 *     +-------+ <-- size
> > > > +	 *     | funcId|
> > > > +	 *     +-------+ <-- size + sizeof(ulong)
> > > > +	 *     | capId |
> > > > +	 *     +-------+ <-- size + 2*sizeof(ulong)
> > > Relying on an undocumented convention that following the region
> > > specified in DeviceTree are two architecture specifically sized integers
> > > isn't good practice.
> > >
> > > This should be covered by the DeviceTree binding, in one way or another.
> > ok. Usually, DTBs don't allow non-hw properties in the dtb. I can try adding
> > a property as cap-id-width if its allowed.
> If you remove the additional part, per the next comment, DeviceTree
> would be oblivious to these properties. I'll don't know if the
> DeviceTree people have any concerns/suggestions about this.
ok.
> > > > +	 */
> > > > +
> > > > +	scmi_info->shmem = devm_ioremap(dev, res.start,
> > > > +					size + 2 * sizeof(unsigned long));
> > > I don't find any code that uses the size of the defined shm, so I don't
> > > think you need to do this dance.
> > Right! I can remove the addition part.
> > > > +	if (!scmi_info->shmem) {
> > > > +		dev_err(dev, "failed to ioremap SCMI Tx shared memory\n");
> > > > +		return -EADDRNOTAVAIL;
> > > > +	}
> > > > +
> > > > +	func_id = readl((void *)(scmi_info->shmem) + size);
> > > > +
> > > > +#ifdef CONFIG_ARM64
> > > > +	cap_id = readq((void *)(scmi_info->shmem) + size +
> > > > +		       sizeof(unsigned long));
> > > > +#else
> > > > +	cap_id = readl((void *)(scmi_info->shmem) + size +
> > > > +		       sizeof(unsigned long));
> > > > +#endif
> > > Please don't make the in-memory representation depend on architecture
> > > specific data types. Quite likely you didn't compile test one of these
> > > variants?
> > >
> > > Just define the in-memory representation as u32 + u64.
> > I tested this for ARM64, I didn't test it for 32bit since Hypervisor doesn't
> > support it currently. In future, it may add 32 bit support too.
> I'd not be surprised if the capability id is 64 bit on a 32-bit machine
> as well, it's not really a property of the architecture.

on 32bit machine, you will have to use SMC32 convention. lt will mean 
that the parameters can only be 32 bit long. If you keep cap-id 64 bit 
in 32bit machines, then it has to be passed in two parameters. Are you 
suggesting, I make this driver dependent on ARM64 and only care about 64 
bit for now?

> But regardless, always using 64 bits in your memory representation will
> at worst waste a few bytes. But the result is a better defined
> interface, and you can avoid the conditional code.
>
> Regards,
> Bjorn