linux-kernel - Re: [PATCHv2 1/4] staging: android: ion: Drop heap type masks

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Message-ID: <20160905112026.GA3173@e106950-lin.cambridge.arm.com>
Date:   Mon, 5 Sep 2016 12:20:27 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Sumit Semwal <sumit.semwal@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Riley Andrews <riandrews@...roid.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        linaro-mm-sig@...ts.linaro.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org,
        Eun Taik Lee <eun.taik.lee@...sung.com>,
        Liviu Dudau <Liviu.Dudau@....com>,
        Jon Medhurst <tixy@...aro.org>,
        Mitchel Humpherys <mitchelh@...eaurora.org>,
        Jeremy Gebben <jgebben@...eaurora.org>,
        Bryan Huntsman <bryanh@...eaurora.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Chen Feng <puck.chen@...ilicon.com>
Subject: Re: [PATCHv2 1/4] staging: android: ion: Drop heap type masks

Hi,

On Fri, Sep 02, 2016 at 12:36:25PM -0700, Laura Abbott wrote:
>On 09/02/2016 06:41 AM, Brian Starkey wrote:
>>Hi Laura,
>>
>>On Thu, Sep 01, 2016 at 03:40:41PM -0700, Laura Abbott wrote:
>>>
>>>There is no advantage to having heap types be a mask. The ion client has
>>>long since dropped the mask. Drop the notion of heap type masks as well.
>>>
>>
>>I know this is the same patch you sent last time, so sorry for not
>>picking this up then - but I'm curious what "The" ion client is here?
>>
>
>ion_client_create used to take a mask to indicate what heap types it
>could allocate from. This hasn't been the case since 2bb9f5034ec7
>("gpu: ion: Remove heapmask from client"). "The ion client" probably
>should have been "struct ion_client"

Ah I see, the in-kernel ion_client. Sorry, I completely forgot that
even existed (because it's totally useless - how is a driver meant to
find the global ion_device?)

>
>>Our ion client(s) certainly still use these masks, and it's still
>>used as a mask within ion itself - even if the relationship between a
>>mask and a heap type has been somewhat lost.
>
>Where is it used in Ion? I don't see it in tree unless I missed something
>and I'm not eager to keep this around for out of tree code. What's the
>actual use for this?

You're certainly right that these heap-ID-to-allocation-mask macros
are unused in the kernel, but I don't really see the reason for
removing them - they are convenient (for now).

Example: I'm using the dummy ion driver, and I want to allocate from
the SYSTEM_CONTIG heap - the ION_HEAP_SYSTEM_CONTIG_MASK gives me the
exact mask I need for that.

It seems your opinion is that heap-IDs are already, and should be,
completely decoupled from their type. That sounds like a good idea to
me, but it's not true (yet) - again check out the dummy driver.

At the moment, heap-IDs are assigned by ion drivers in any way they
see fit. For as long as that stays the case there's always going to
be heap-masks hard-coded in UAPI kernel headers (in-tree or not), so
removing these particular masks seems a bit fruitless.

I'd rather see driver-assigned heap-IDs disappear completely, and have
them assigned by ion core from an idr or something. At that point
these macros really *are* meaningless, and I'd be totally fine with
removing them (and userspace won't be able to depend on hard-coded
allocation masks any more - it will have to use the query ioctl,
which I assume is the whole point?).

IMO it's not the right time to remove these macros, because they still
have meaning and usefulness.

Cheers,
Brian

>
>>
>>Thanks,
>>Brian
>>
>>>Signed-off-by: Laura Abbott <labbott@...hat.com>
>>>---
>>>drivers/staging/android/uapi/ion.h | 6 ------
>>>1 file changed, 6 deletions(-)
>>>
>>>diff --git a/drivers/staging/android/uapi/ion.h b/drivers/staging/android/uapi/ion.h
>>>index 0a8e40f..a9c4e8b 100644
>>>--- a/drivers/staging/android/uapi/ion.h
>>>+++ b/drivers/staging/android/uapi/ion.h
>>>@@ -44,14 +44,8 @@ enum ion_heap_type {
>>>                   * must be last so device specific heaps always
>>>                   * are at the end of this enum
>>>                   */
>>>-    ION_NUM_HEAPS = 16,
>>>};
>>>
>>>-#define ION_HEAP_SYSTEM_MASK        (1 << ION_HEAP_TYPE_SYSTEM)
>>>-#define ION_HEAP_SYSTEM_CONTIG_MASK    (1 << ION_HEAP_TYPE_SYSTEM_CONTIG)
>>>-#define ION_HEAP_CARVEOUT_MASK        (1 << ION_HEAP_TYPE_CARVEOUT)
>>>-#define ION_HEAP_TYPE_DMA_MASK        (1 << ION_HEAP_TYPE_DMA)
>>>-
>>>#define ION_NUM_HEAP_IDS        (sizeof(unsigned int) * 8)
>>>
>>>/**
>>>--
>>>2.7.4
>>>
>