linux-kernel - Re: [PATCH v2 02/15] NTB: Set dma mask and dma coherent mask to NTB devices

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Date:   Tue, 5 Dec 2017 10:21:58 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jon Mason <jdmason@...zu.us>, Serge Semin <fancer.lancer@...il.com>
Cc:     Dave Jiang <dave.jiang@...el.com>,
        "Hubbe, Allen" <Allen.Hubbe@....com>,
        "S-k, Shyam-sundar" <Shyam-sundar.S-k@....com>,
        "Yu, Xiangliang" <Xiangliang.Yu@....com>,
        Gary R Hook <gary.hook@....com>, Sergey.Semin@...latforms.ru,
        linux-ntb <linux-ntb@...glegroups.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 02/15] NTB: Set dma mask and dma coherent mask to NTB
 devices



On 05/12/17 09:51 AM, Jon Mason wrote:
> On Sun, Dec 3, 2017 at 2:17 PM, Serge Semin <fancer.lancer@...il.com> wrote:
>> The dma_mask and dma_coherent_mask fields of the NTB struct device
>> weren't initialized in hardware drivers. In fact it should be done
>> instead of PCIe interface usage, since NTB clients are supposed to
>> use NTB API only and left unaware of real hardware implementation.
>> In addition to that ntb_device_register() method shouldn't clear
>> the passed ntb_dev structure, since it dma_mask is initialized
>> by hardware drivers.
> 
> This seems like a bug fix.  Should this be broken out of this patch or
> is it not noticed without your changes?

I was digging into this a bit more and in addition to my earlier review 
comments I think the patch is incorrect for a couple reasons:

1) dma_set_mask() (and friends) are meant to inform the kernel of device 
specific restrictions it cannot know. Calling it with the parent PCI 
device's mask seems pointless.

2) Every dma_alloc() call in the NTB subsystem uses the PCI device's 
struct device. This is ugly in itself but until it changes this patch 
does nothing.

Logan