Re: [PATCH] media: venus: firmware: Use of_reserved_mem_lookup()

[Stephan Gerhold <stephan@gerhold.net>]

On Wed, May 31, 2023 at 11:36:52AM +0530, Vikash Garodia wrote:
> On 5/29/2023 11:46 PM, Stephan Gerhold wrote:
> > Reserved memory can be either looked up using the generic function
> > of_address_to_resource() or using the special of_reserved_mem_lookup().
> > The latter has the advantage that it ensures that the referenced memory
> > region was really reserved and is not e.g. status = "disabled".
> > 
> > of_reserved_mem also supports allocating reserved memory dynamically at
> > boot time. This works only when using of_reserved_mem_lookup() since
> > there won't be a fixed address in the device tree.
> IIUC, this would avoid precomputing the hard range for different firmware
> regions and also make it more flexible to adjust the sizes, if anyone wants a
> bigger size later.
> Incase a specific firmware needs a dedicate start address, do we have an option
> to specify the same ?
> 

If you want a specific start address (or in other words: a fixed base
address) then you should continue using static reservation for that
component. You can mix static and dynamic reservations. The static ones
(with fixed addresses) will be reserved first, then the dynamic ones
will be allocated from the free space.

I have this example for one device in my proposal at [1]:

	/* Firmware must be loaded at address 0x8b600000 */
	wcnss_mem: wcnss@8b600000 {
		reg = <0x8b600000 0x600000>;
		no-map;
	};
	/* Firmware can be loaded anywhere with 1 MiB alignment */
	venus_mem: venus {
		size = <0x500000>;
		alignment = <0x100000>;
		no-map;
	};

The wcnss_mem will be always at 0x8b600000, but the venus_mem can be
loaded somewhere around that. If only certain regions need a fixed
address this still provides the flexibility to change sizes more easily.

Does that answer your question? I wasn't sure what exactly you mean with
a "dedicated start address". :)

Thanks,
Stephan

[1]: https://lore.kernel.org/linux-arm-msm/20230510-dt-resv-bottom-up-v1-5-3bf68873dbed@gerhold.net/