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From:   "Valdis Kl=?utf-8?Q?=c4=93?=tnieks" <valdis.kletnieks@vt.edu>
To:     Pali =?utf-8?B?Um9ow6Fy?= <pali.rohar@gmail.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        devel@driverdev.osuosl.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Sasha Levin <alexander.levin@microsoft.com>,
        Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH] staging: exfat: add exfat filesystem code to staging
In-Reply-To: <20190829121435.bsl5cnx7yqgakpgb@pali>
References: <20190828160817.6250-1-gregkh@linuxfoundation.org>
 <20190829121435.bsl5cnx7yqgakpgb@pali>
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Date:   Thu, 29 Aug 2019 08:34:04 -0400
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On Thu, 29 Aug 2019 14:14:35 +0200, Pali Roh=E1r said:
> On Wednesday 28 August 2019 18:08:17 Greg Kroah-Hartman wrote:
> > The full specification of the filesystem can be found at:
> >   https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specifi=
cation
>
> This is not truth. This specification is not =22full=22. There are miss=
ing
> important details, like how is TexFAT implemented.=20

Well..given that the spec says it's an extension used by Windows CE...

> 1.5 Windows CE and TexFAT

> TexFAT is an extension to exFAT that adds transaction-safe operational
> semantics on top of the base file system. TexFAT is used by Windows CE.=
 TexFAT
> requires the use of the two FATs and allocation bitmaps for use in
> transactions. It also defines several additional structures including p=
adding
> descriptors and security descriptors.

And these two pieces of info:

> 3.1.13.1 ActiveFat Field

> The ActiveFat field shall describe which FAT and Allocation Bitmap are =
active
> (and implementations shall use), as follows:

> 0, which means the First FAT and First Allocation Bitmap are active

> 1, which means the Second FAT and Second Allocation Bitmap are active a=
nd is
> possible only when the NumberOfFats field contains the value 2

> Implementations shall consider the inactive FAT and Allocation Bitmap a=
s stale.
> Only TexFAT-aware implementations shall switch the active FAT and Alloc=
ation
> Bitmaps (see Section 7.1).

> 3.1.16 NumberOfFats Field
> The NumberOfFats field shall describe the number of FATs and Allocation=
 Bitmaps
> the volume contains.

> The valid range of values for this field shall be:

> 1, which indicates the volume only contains the First FAT and First All=
ocation Bitmap

> 2, which indicates the volume contains the First FAT, Second FAT, First=

> Allocation Bitmap, and Second Allocation Bitmap; this value is only val=
id for
> TexFAT volumes

I think we're OK if we just set ActiveFat to 0 and NumberOfFats to 1.

Unless somebody has actual evidence of a non-WindowsCE extfat that has
NumberOfFats =3D=3D 2....

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