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Date:   Thu, 6 Apr 2023 18:06:04 +0200
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Subject: Re: [PATCH 2/2] of: fdt: Allow the kernel to mark nomap regions
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To:     Rob Herring <robh+dt@kernel.org>, Lucas Tanure <tanure@linux.com>
Cc:     Frank Rowand <frowand.list@gmail.com>,
        Mike Rapoport <rppt@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, jbrunet@baylibre.com,
        linux-amlogic@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org,
        martin.blumenstingl@googlemail.com, narmstrong@baylibre.com,
        stefan@agner.ch
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From:   Neil Armstrong <neil.armstrong@linaro.org>
Organization: Linaro Developer Services
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On 06/04/2023 17:48, Rob Herring wrote:
> On Thu, Apr 6, 2023 at 10:14 AM Lucas Tanure <tanure@linux.com> wrote:
>>
>> Reserved regions can be described in FDT and device trees, but FDT doesn't
>> provide the related flags, like nomap.
> 
> It took me a minute to understand what you meant by FDT vs. device
> trees. Use the exact things you are talking about: /memreserve/ and
> /reserved-memory node.
> 
>> So allow the kernel to mark regions where the base and size received from
>> the device tree are the same as the base and region on FDT.
>> Here we trust that the device tree has a more updated description of the
>> region than the one received from FDT.
>>
>> Signed-off-by: Lucas Tanure <tanure@linux.com>
>> ---
>>   drivers/of/fdt.c | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
>> index d1a68b6d03b3..754a7ea4f45c 100644
>> --- a/drivers/of/fdt.c
>> +++ b/drivers/of/fdt.c
>> @@ -482,11 +482,13 @@ static int __init early_init_dt_reserve_memory(phys_addr_t base,
>>          if (nomap) {
>>                  /*
>>                   * If the memory is already reserved (by another region), we
>> -                * should not allow it to be marked nomap, but don't worry
>> -                * if the region isn't memory as it won't be mapped.
>> +                * should not allow it to be marked nomap, unless is the exact same region
>> +                * (same base and size), which the kernel knows better and should be allowed to mark
>> +                *  it as nomap.
>> +                * But don't worry if the region isn't memory as it won't be mapped.
>>                   */
>> -               if (memblock_overlaps_region(&memblock.memory, base, size) &&
>> -                   memblock_is_region_reserved(base, size))
>> +               if (memblock_overlaps_region(&memblock.memory, base, size) == MEMBLOCK_OVERLAPS &&
>> +                   memblock_is_region_reserved(base, size) == MEMBLOCK_OVERLAPS)
> 
> Won't this fail to work as IIRC memblock will merge regions when they
> are adjacent and have the same atrributes.
> 
> Perhaps instead, the DT code should ignore any /memreserve/ entries
> that are also in /reserved-memory.
> 
> I would suggest just reverse the order they are processed, but I
> suspect that might cause some regression. This code is all fragile
> especially with platforms putting in 100 regions.
> 
> Finally, perhaps fix u-boot. The reason the reserved location goes in
> both places was to support an OS not supporting /reserved-memory. I
> think that support has been in place for a lot longer than anyone
> would care about.

Fixing U-Boot won't fix already tagged and in-the-field mainline u-boot
releases that adds /memreserve/ entries, so yes u-boot should be definitely
fixed but Linux should ignore the /memreserve/ entries when they matches
an /reserved-memory node like when the U-Boot /memreserve/ code was added.

Neil

> 
> Rob
> 
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