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Subject: Re: [PATCH] uio: add UIO_MEM_CUSTOM support
To: Xiubo Li <lixiubo@cmss.chinamobile.com>,
        Greg KH <gregkh@linuxfoundation.org>
References: <1487133786-25545-1-git-send-email-lixiubo@cmss.chinamobile.com>
 <20170215171937.GA4705@kroah.com>
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Cc: mchristi@redhat.com, namei.unix@gmail.com,
        linux-kernel@vger.kernel.org
From: Andy Grover <agrover@redhat.com>
Organization: Red Hat
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Date: Wed, 15 Feb 2017 21:01:52 -0800
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On 02/15/2017 05:34 PM, Xiubo Li wrote:
>>> --- a/drivers/uio/uio.c
>>> +++ b/drivers/uio/uio.c
>>> @@ -708,6 +708,8 @@ static int uio_mmap(struct file *filep, struct
>>> vm_area_struct *vma)
>>>           case UIO_MEM_LOGICAL:
>>>           case UIO_MEM_VIRTUAL:
>>>               return uio_mmap_logical(vma);
>>> +        case UIO_MEM_CUSTOM:
>>> +            return 0;
>> How does this help?
> For example, the TCMU will use the map area as ISCSI commands & data ring
> buffer(uio0 --> map0). Currently the TCMU will using the fixed small
> size map
> area as the ring buffer, but this will be the bottleneck for high iops.
> 
> Without knowing how large it is enough, so the new scheme will use the
> fixed
> small ring buffer area(about 64M ~ 128M) + dynamically "growing" ring
> buffer
> area(about 1.5G).

The following code is in uio_mmap() in uio.c:

if (idev->info->mmap) {
	ret = idev->info->mmap(idev->info, vma);
	return ret;
}

switch (idev->info->mem[mi].memtype) {
	case UIO_MEM_PHYS:
		return uio_mmap_physical(vma);
	case UIO_MEM_LOGICAL:
	case UIO_MEM_VIRTUAL:
		return uio_mmap_logical(vma);
	default:
		return -EINVAL;
}

We already have the equivalent of a CUSTOM memtype because TCMU sets the
info->mmap fn, overriding uio's default handling choices in favor of its
own.

HTH -- Regards -- Andy