Re: use of malloc(9) for DMA buffers

[Michael Lorenz <macallan%netbsd.org@localhost>]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hello,

On Aug 11, 2008, at 8:17 AM, Marco Trillo wrote:

> I have a doubt regarding dbdma.c dbdma_alloc(). It uses malloc(9) to
> allocate the buffer for the DMA command descriptors:
>
>   122: dbdma_command_t        *
>   123: dbdma_alloc(size)
>   124:        int size;
>   125: {
>   126:        u_int buf;
>   127:
>   128:        buf = (u_int)malloc(size + 0x0f, M_DEVBUF, M_WAITOK);
>   129:        buf = (buf + 0x0f) & ~0x0f;
>   130:
>   131:        return (dbdma_command_t *)buf;
>   132: }
>
> Since these descriptors are directly accessed by hardware, wouldn't
> this cause problems if the buffer crosses a page boundary and so it's
> not contiguous in physical memory? Or does this never happen?

On macppc that's safe ( kernel virtual addresses are identical to  
physical addresses ) but on most other platforms it would likely blow  
up in your face.

have fun
Michael

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.7 (Darwin)

iQEVAwUBSKAzQcpnzkX8Yg2nAQJboQgAiLWINL5lfcpCXoFrWt9bb3vhKld2WjQX
0G/UjWRSX6vuH2TUCsJCOAWw9xk+1cuaDLXyE8cIdjoIquWRodhb2Oj0xkq7urJv
++8vSzBnFfspArLjhm1/3Eb4t7x7F62wYbZmsGe4v96z1sfJ9LXOXt1thpMA6qPK
R6pxXNMbQz3uv2M3JX6PJ1Tq6uI1dCMX3KBcQQL6RVx5HrKlSMFito7gnNK57uSv
vKqfHcOfpqxL8G/stOm0p4B8dIP0dq40pGBv6E7ClERuasY0txHVFfNnnDlmYeEa
N7aWu9WJYpY9H/8lBK5MXOdubfKJgoOKmTpxztblVgQ6CcuE8szEpQ==
=ZnV7
-----END PGP SIGNATURE-----