Re: [PATCH v10 05/11] arm: armv8: mmu: add mem_map_from_dram_banks

[Anshul Dalal <anshuld@ti.com>]

Hello Ilias,

On Fri Oct 17, 2025 at 5:39 PM IST, Ilias Apalodimas wrote:
> Hi Anshul,
>
> On Fri, 10 Oct 2025 at 16:44, Anshul Dalal <anshuld@ti.com> wrote:
>>
>> For armv8, U-Boot uses a static map defined as 'mem_map' for configuring
>> the MMU as part of mmu_setup.
>>
>> But since the exact configuration of memory banks might not be known at
>> build time, many platforms such as imx9, versal2 etc. utilize
>> gd->bd->bi_dram to configure the static map at runtime.
>>
>> Therefore this patch adds a new API mem_map_from_dram_banks that
>> modifies the static map in a similar way. Allowing the caller to map all
>> dram banks by just passing the index to last entry in their mem_map and
>> it's length.
>>
>> Reviewed-by: Dhruva Gole <d-gole@ti.com>
>> Signed-off-by: Anshul Dalal <anshuld@ti.com>
>> Tested-by: Wadim Egorov <w.egorov@phytec.de>
>> ---
>>  arch/arm/cpu/armv8/cache_v8.c    | 28 ++++++++++++++++++++++++++++
>>  arch/arm/include/asm/armv8/mmu.h | 11 +++++++++++
>>  2 files changed, 39 insertions(+)
>>
>> diff --git a/arch/arm/cpu/armv8/cache_v8.c b/arch/arm/cpu/armv8/cache_v8.c
>> index 74c78cb2fb0..9b3c37dae82 100644
>> --- a/arch/arm/cpu/armv8/cache_v8.c
>> +++ b/arch/arm/cpu/armv8/cache_v8.c
>> @@ -58,6 +58,34 @@ static int get_effective_el(void)
>>         return el;
>>  }
>>
>> +int mem_map_from_dram_banks(unsigned int index, unsigned int len, u64 attrs)
>> +{
>> +       unsigned int i;
>> +       int ret = fdtdec_setup_memory_banksize();
>> +
>> +       if (ret) {
>> +               log_err("%s: Failed to setup dram banks\n", __func__);
>> +               return ret;
>> +       }
>> +
>> +       if (index + CONFIG_NR_DRAM_BANKS >= len) {
>> +               log_err("%s: Provided mem_map array has insufficient size for DRAM entries\n",
>> +                       __func__);
>> +               return -ENOMEM;
>> +       }
>> +
>> +       for (i = 0; i < CONFIG_NR_DRAM_BANKS; i++) {
>> +               mem_map[index].virt = gd->bd->bi_dram[i].start;
>> +               mem_map[index].phys = gd->bd->bi_dram[i].start;
>> +               mem_map[index].size = gd->bd->bi_dram[i].size;
>> +               mem_map[index].attrs = attrs;
>> +               index++;
>> +       }
>> +
>> +       memset(&mem_map[index], 0, sizeof(mem_map[index]));
>> +
>> +       return 0;
>> +}
>
> Is there a reason we add all the memory maps now and remove them later
> with mmu_unmap_reserved_mem(). IOW we could add the DT parsing in this
> function and not map the 'no-map' addresses to begin with.
> Unless someone changes the DTB on the fly ?
>

There are two reasons for doing it this way:

1. Modifying mem_map directly makes creating multiple carveouts quite
complex with the logic for handling all the edge cases, for example see
v4[1] of this series. It's much simpler to use the MMU APIs instead
after calling mmu_setup with everything mapped in mem_map.

2. We can't blindly unmap all 'no-map' regions since some regions that
would be 'no-map' in the kernel's context are required at U-Boot, like
the memory pools for remote cores that rely on U-Boot to load their
firmware.

Regards,
Anshul