Re: dma_declare_coherent_memory fails for RAM allocated memory

[Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>]

Hi,

On  1.06.2016 14:18, Brian Starkey wrote:
> Hi Ivo,
>
> On Sun, May 29, 2016 at 05:56:02PM +0300, Ivaylo Dimitrov wrote:
>> Hi,
>>
>> When trying to declare and use DT reserved memory region on ARM
>> (OMAP3), dma_declare_coherent_memory() fails in memremap(). This is
>> from today's master:
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 0 PID: 1 at kernel/memremap.c:111 memremap+0x118/0x194
>> memremap attempted on ram 0x8f800000 size: 0x700000
>> Modules linked in:
>> CPU: 0 PID: 1 Comm: swapper Not tainted 4.6.0+ #15
>> Hardware name: Nokia RX-51 board
>> [<c010bc30>] (unwind_backtrace) from [<c0109f70>] (show_stack+0x10/0x14)
>> [<c0109f70>] (show_stack) from [<c0126d80>] (__warn+0xcc/0xf8)
>> [<c0126d80>] (__warn) from [<c0126e40>] (warn_slowpath_fmt+0x34/0x44)
>> [<c0126e40>] (warn_slowpath_fmt) from [<c019c8d4>] (memremap+0x118/0x194)
>> [<c019c8d4>] (memremap) from [<c03b88fc>]
>> (dma_init_coherent_memory+0x48/0x104)
>> [<c03b88fc>] (dma_init_coherent_memory) from [<c03b89e4>]
>> (dma_declare_coherent_memory+0x2c/0x68)
>> [<c03b89e4>] (dma_declare_coherent_memory) from [<c0115c0c>]
>> (rmem_omapfb_device_init+0x34/0x64)
>> [<c0115c0c>] (rmem_omapfb_device_init) from [<c045b0f8>]
>> (of_reserved_mem_device_init+0x94/0xd8)
>> [<c045b0f8>] (of_reserved_mem_device_init) from [<c080aaf8>]
>> (omapdss_init_of+0xe4/0x154)
>> [<c080aaf8>] (omapdss_init_of) from [<c08033f4>]
>> (customize_machine+0x20/0x44)
>> [<c08033f4>] (customize_machine) from [<c01016b8>]
>> (do_one_initcall+0xac/0x158)
>> [<c01016b8>] (do_one_initcall) from [<c0800d64>]
>> (kernel_init_freeable+0xf8/0x1c8)
>> [<c0800d64>] (kernel_init_freeable) from [<c05519dc>]
>> (kernel_init+0x8/0x110)
>> [<c05519dc>] (kernel_init) from [<c0107258>] (ret_from_fork+0x14/0x3c)
>> ---[ end trace 73a8c076df72166b ]---
>> omapfb: dma_declare_coherent_memory failed
>>
>>
>> The failing code looks like:
>> .
>> .
>> .
>> static int rmem_omapfb_device_init(struct reserved_mem *rmem, struct
>> device *dev)
>> {
>>     int dma;
>>
>>     if (rmem->priv)
>>         return 0;
>>
>>     dma = dma_declare_coherent_memory(&omap_fb_device.dev, rmem->base,
>>                       rmem->base, rmem->size,
>>                       DMA_MEMORY_MAP |
>>                       DMA_MEMORY_EXCLUSIVE);
>>     if (!(dma & DMA_MEMORY_MAP)) {
>>             pr_err("omapfb: dma_declare_coherent_memory failed\n");
>>             return -ENOMEM;
>>     }
>>     else
>>         rmem->priv = omap_fb_device.dev.dma_mem;
>>
>>     return 0;
>> }
>>
>> static void rmem_omapfb_device_release(struct reserved_mem *rmem,
>>                        struct device *dev)
>> {
>>     dma_release_declared_memory(&omap_fb_device.dev);
>> }
>>
>> static const struct reserved_mem_ops rmem_omapfb_ops = {
>>     .device_init    = rmem_omapfb_device_init,
>>     .device_release = rmem_omapfb_device_release,
>> };
>>
>> static int __init rmem_omapfb_setup(struct reserved_mem *rmem)
>> {
>>     rmem->ops = &rmem_omapfb_ops;
>>     pr_info("omapfb: reserved %d bytes at %pa\n", rmem->size,
>> &rmem->base);
>>
>>     return 0;
>> }
>>
>> RESERVEDMEM_OF_DECLARE(dss, "ti,omapfb-memsize", rmem_omapfb_setup);
>>
>>
>> It turns out that dma_init_coherent_memory calls memremap with
>> MEMREMAP_WC flag, which is disallowed for RAM IIUC.
>>
>
> Right. If you want to use a memory region as coherent DMA memory, then
> that same region can't be System RAM, because System RAM is usually
> mapped in a non-coherent fashion.
>
>> I quickly hacked some code to fix the issue, but as memremap API is
>> relatively new(esp to me), I wonder if this is the correct way to go:
>>
>> diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
>> index bdf28f7..04b1687 100644
>> --- a/drivers/base/dma-coherent.c
>> +++ b/drivers/base/dma-coherent.c
>> @@ -5,6 +5,7 @@
>> #include <linux/io.h>
>> #include <linux/slab.h>
>> #include <linux/kernel.h>
>> +#include <linux/memblock.h>
>> #include <linux/module.h>
>> #include <linux/dma-mapping.h>
>>
>> @@ -32,8 +33,12 @@ static bool dma_init_coherent_memory(
>>        if (!size)
>>                goto out;
>>
>> -       if (flags & DMA_MEMORY_MAP)
>> -               mem_base = memremap(phys_addr, size, MEMREMAP_WC);
>> +       if (flags & DMA_MEMORY_MAP) {
>> +               unsigned long map_type =
>> memblock_is_map_memory(phys_addr) ?
>> +                                                MEMREMAP_WB :
>> MEMREMAP_WC;
>> +
>> +               mem_base = memremap(phys_addr, size, map_type);
>> +       }
>>        else
>>                mem_base = ioremap(phys_addr, size);
>>        if (!mem_base)
>>
>> Does the above code looks sane? How to fix the problem if not?
>
> AFAIK dma_declare_coherent_memory() should only be used on physical
> addresses which don't already have a CPU virtual mapping. Certainly
> returning (potentially) cached memory as DMA coherent is asking for
> trouble.
>
> I think what you want to do is add a "no-map;" property to your
> reserved-memory region in DT. That will stop the kernel from using it
> as System RAM, leaving it free for your framebuffer.
>

I tried to use "no-map", but for some reason it doesn't allow me to 
allocate more than 2MB, anything above results in:

qemu: fatal: Trying to execute code outside RAM or ROM at 0xffff0010

R00=22222220 R01=cf901000 R02=cfaffff4 R03=e0000000
R04=0008ff80 R05=00090000 R06=00000000 R07=c096333c
R08=c0904dc0 R09=c093a23c R10=00000001 R11=c0727b3b
R12=0000ff80 R13=c093af8c R14=c082aba0 R15=ffff0010
PSR=a00001d7 N-C- A abt32
Aborted (core dumped)

I was able to track the code to memblock_isolate_range() - if I return 
-ENOMEM on http://lxr.free-electrons.com/source/mm/memblock.c#L671, the 
above crash does not occur(however the allocation fails as well). 
Putting some printks doesn't reveal anything suspicious to me:

for 2MB:

[    0.000000] base 8fe00000, size 200000, rgn->base 80000000, rgn->size 
10000000, type->total_size 10000000
[    0.000000] base - rbase fe00000


for 4MB:

[    0.000000] base 8fc00000, size 400000, rgn->base 80000000, rgn->size 
10000000, type->total_size 10000000
[    0.000000] base - rbase fc00000


However, this is where my limited memblock knowledge ends, so any help 
is appreciated.

Maybe I should CC memblock authors as well?

Thanks,
Ivo