Re: [PATCH] of: reserved_mem: Allow reserved_mem framework detect "cma=" kernel param

[Oreoluwa Babatunde]

Hi Rob,

On 12/10/2025 6:07 AM, Rob Herring wrote:
> On Tue, Dec 9, 2025 at 6:20 PM Oreoluwa Babatunde
> <oreoluwa.babatunde@xxxxxxxxxxxxxxxx> wrote:
>>
>> When initializing the default cma region, the "cma=" kernel parameter
>> takes priority over a DT defined linux,cma-default region. Hence, give
>> the reserved_mem framework the ability to detect this so that the DT
>> defined cma region can skip initialization accordingly.
> 
> Please explain here why this is a new problem. Presumably the
> RESERVEDMEM_OF_DECLARE hook after commit xxxx gets called before the
> early_param hook. And why is it now earlier?

ACK. I will add more of this info in the next patch version.

> 
> I don't really like the state/ordering having to be worried about in 2 places.

The advantage to having the state visible to the reserved_mem code is that
we can skip adding the DT node to the resrved_mem array since it actually
won't be used.

If this is still not preferred, another option would be to use a helper
function  in contiguous.c to call dma_contiguous_early_fixup() for the
reserved_mem code. This way, "size_cmdline" can be checked internally within
the file and it can call dma_contiguous_early_fixup based on that.

> 
>> Signed-off-by: Oreoluwa Babatunde <oreoluwa.babatunde@xxxxxxxxxxxxxxxx>
>> ---
>>  drivers/of/of_reserved_mem.c | 19 +++++++++++++++++--
>>  include/linux/cma.h          |  1 +
>>  kernel/dma/contiguous.c      | 16 ++++++++++------
>>  3 files changed, 28 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/of/of_reserved_mem.c b/drivers/of/of_reserved_mem.c
>> index 2e9ea751ed2d..bef68a4916b5 100644
>> --- a/drivers/of/of_reserved_mem.c
>> +++ b/drivers/of/of_reserved_mem.c
>> @@ -158,7 +158,7 @@ static int __init __reserved_mem_reserve_reg(unsigned long node,
>>         phys_addr_t base, size;
>>         int len;
>>         const __be32 *prop;
>> -       bool nomap;
>> +       bool nomap, default_cma;
>>
>>         prop = of_get_flat_dt_prop(node, "reg", &len);
>>         if (!prop)
>> @@ -171,6 +171,12 @@ static int __init __reserved_mem_reserve_reg(unsigned long node,
>>         }
>>
>>         nomap = of_get_flat_dt_prop(node, "no-map", NULL) != NULL;
>> +       default_cma = of_get_flat_dt_prop(node, "linux,cma-default", NULL);
>> +
>> +       if (default_cma && cma_skip_dt_default_reserved_mem()) {
>> +               pr_err("Skipping dt linux,cma-default for \"cma=\" kernel param.\n");
>> +               return -EINVAL;
>> +       }
>>
>>         while (len >= t_len) {
>>                 base = dt_mem_next_cell(dt_root_addr_cells, &prop);
>> @@ -256,12 +262,15 @@ void __init fdt_scan_reserved_mem_reg_nodes(void)
>>
>>         fdt_for_each_subnode(child, fdt, node) {
>>                 const char *uname;
>> +               bool default_cma = of_get_flat_dt_prop(child, "linux,cma-default", NULL);
>>
>>                 prop = of_get_flat_dt_prop(child, "reg", &len);
>>                 if (!prop)
>>                         continue;
>>                 if (!of_fdt_device_is_available(fdt, child))
>>                         continue;
>> +               if (default_cma && cma_skip_dt_default_reserved_mem())
>> +                       continue;
>>
>>                 uname = fdt_get_name(fdt, child, NULL);
>>                 if (len && len % t_len != 0) {
>> @@ -406,7 +415,7 @@ static int __init __reserved_mem_alloc_size(unsigned long node, const char *unam
>>         phys_addr_t base = 0, align = 0, size;
>>         int len;
>>         const __be32 *prop;
>> -       bool nomap;
>> +       bool nomap, default_cma;
>>         int ret;
>>
>>         prop = of_get_flat_dt_prop(node, "size", &len);
>> @@ -430,6 +439,12 @@ static int __init __reserved_mem_alloc_size(unsigned long node, const char *unam
>>         }
>>
>>         nomap = of_get_flat_dt_prop(node, "no-map", NULL) != NULL;
>> +       default_cma = of_get_flat_dt_prop(node, "linux,cma-default", NULL);
>> +
>> +       if (default_cma && cma_skip_dt_default_reserved_mem()) {
>> +               pr_err("Skipping dt linux,cma-default for \"cma=\" kernel param.\n");
>> +               return -EINVAL;
>> +       }
>>
>>         /* Need adjust the alignment to satisfy the CMA requirement */
>>         if (IS_ENABLED(CONFIG_CMA)
>> diff --git a/include/linux/cma.h b/include/linux/cma.h
>> index 62d9c1cf6326..3d3047029950 100644
>> --- a/include/linux/cma.h
>> +++ b/include/linux/cma.h
>> @@ -47,6 +47,7 @@ extern int cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
>>                                         unsigned int order_per_bit,
>>                                         const char *name,
>>                                         struct cma **res_cma);
>> +extern bool cma_skip_dt_default_reserved_mem(void);
>>  extern struct page *cma_alloc(struct cma *cma, unsigned long count, unsigned int align,
>>                               bool no_warn);
>>  extern bool cma_pages_valid(struct cma *cma, const struct page *pages, unsigned long count);
>> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
>> index d9b9dcba6ff7..9071c08650e3 100644
>> --- a/kernel/dma/contiguous.c
>> +++ b/kernel/dma/contiguous.c
>> @@ -90,6 +90,16 @@ static int __init early_cma(char *p)
>>  }
>>  early_param("cma", early_cma);
>>
>> +/*
>> + * cma_skip_dt_default_reserved_mem - This is called from the
>> + * reserved_mem framework to detect if the default cma region is being
>> + * set by the "cma=" kernel parameter.
>> + */
>> +bool __init cma_skip_dt_default_reserved_mem(void)
>> +{
>> +       return size_cmdline != -1;
>> +}
>> +
>>  #ifdef CONFIG_DMA_NUMA_CMA
>>
>>  static struct cma *dma_contiguous_numa_area[MAX_NUMNODES];
>> @@ -463,12 +473,6 @@ static int __init rmem_cma_setup(struct reserved_mem *rmem)
>>         struct cma *cma;
>>         int err;
>>
>> -       if (size_cmdline != -1 && default_cma) {
>> -               pr_info("Reserved memory: bypass %s node, using cmdline CMA params instead\n",
>> -                       rmem->name);
>> -               return -EBUSY;
>> -       }
>> -
>>         if (!of_get_flat_dt_prop(node, "reusable", NULL) ||
>>             of_get_flat_dt_prop(node, "no-map", NULL))
>>                 return -EINVAL;
>> --
>> 2.34.1
>>
>>

Regards,
Oreoluwa