About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4)

About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4)

[yjg0107]

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Hi Luiz,

Question is same as https://github.com/bluez/bluez/issues/2073.
About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4), you said that this will be fixed to enable user space changed, from #201 (comment).
The patch commit is torvalds/linux@09572fc, right?

However, why HCI_CHANNEL_USER type can not change MTU value? Google bluetooth aidl uses HCI_CHANNEL_USER type from https://cs.android.com/android/platform/superproject/+/android-latest-release:hardware/interfaces/bluetooth/aidl/default/net_bluetooth_mgmt.cpp line 282.

Could kernel remove the limitation that HCI_CHANNEL_USER can not change MTU?

Thanks.
B.R.
Jigong Yin



发自vivo电子邮件

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回复: About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4)

[yjg0107]

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Hi Vudentz,

In the future, BT SIG will introduce the High Data Throughput (HDT), which may require transmit larger data size, not just 1024.
So what's your opinions? Could kernel remove the limitation that HCI_CHANNEL_USER can not change MTU?
Looking forward to your reply, thank you very much.

B.R.
Jigong Yin(殷吉功)---------- 回复的原邮件 ----------
发件人：yjg0107 <yjg0107@163.com>
日期：2026年4月27日 14:40
主题：About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4)
收件人："luiz.dentz" <luiz.dentz@gmail.com>
抄送：linux-bluetooth <linux-bluetooth@vger.kernel.org>

About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4)
Hi Luiz,

Question is same as https://github.com/bluez/bluez/issues/2073.
About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4), you said that this will be fixed to enable user space changed, from #201 (comment).
The patch commit is torvalds/linux@09572fc, right?

However, why HCI_CHANNEL_USER type can not change MTU value? Google bluetooth aidl uses HCI_CHANNEL_USER type from https://cs.android.com/android/platform/superproject/+/android-latest-release:hardware/interfaces/bluetooth/aidl/default/net_bluetooth_mgmt.cpp line 282.

Could kernel remove the limitation that HCI_CHANNEL_USER can not change MTU?

Thanks.
B.R.
Jigong Yin



发自vivo电子邮件

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Re: 转发: 回复: About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4)

[Chris Lu (陸稚泓)]

On Wed, 2026-05-20 at 15:03 +0800, yjg0107 wrote:

Hi Luiz,

I received a question about the restrictions of HCI_CHANNEL_USER and
HCI_MAX_FRAME_SIZE. Since this involves changes related to Kernel HCI
layer, what's your view on relaxing HCI_MAX_FRAME_SIZE limit?

I saw that there was some discussion about this on bluez github, but it
seems there hasn't been any activity for a while. Would you suggest
they directly upload a patch they want and continue the discussion with
you?

> 
> ---------- 转发的邮件 ----------
> 发件人：yjg0107 <yjg0107@163.com>
> 日期：2026年5月7日 17:55
> 主题：回复: About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4)
> 收件人："luiz.dentz" <luiz.dentz@gmail.com>
> 抄送：linux-bluetooth <linux-bluetooth@vger.kernel.org>
> 
> 
> 
> 
> 
> 
> 
> 
> Hi Vudentz,
> 
> In the future, BT SIG will introduce the High Data Throughput (HDT),
> which may require transmit larger data size, not just 1024.
> So what's your opinions? Could kernel remove the limitation that
> HCI_CHANNEL_USER can not change MTU?
> Looking forward to your reply, thank you very much.
> 
> B.R.
> Jigong Yin(殷吉功)
> ---------- 回复的原邮件 ----------
> 发件人：yjg0107 <yjg0107@163.com>
> 日期：2026年4月27日 14:40
> 主题：About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4)
> 收件人："luiz.dentz" <luiz.dentz@gmail.com>
> 抄送：linux-bluetooth <linux-bluetooth@vger.kernel.org>
> 
> 
> 
> 
> 
> 
> 
> 
> Hi Luiz,
> 
> Question is same as https://github.com/bluez/bluez/issues/2073.
> About Kernel limitation HCI_MAX_FRAME_SIZE (1024 + 4), you said that
> this will be fixed to enable user space changed, from #201 (comment).
> The patch commit is torvalds/linux@09572fc, right?
> 
> However, why HCI_CHANNEL_USER type can not change MTU value? Google
> bluetooth aidl uses HCI_CHANNEL_USER type
> fromhttps://cs.android.com/android/platform/superproject/+/android-la
> test-
> release:hardware/interfaces/bluetooth/aidl/default/net_bluetooth_mgmt
> .cpp line 282.
> 
> Could kernel remove the limitation that HCI_CHANNEL_USER can not
> change MTU?
> 
> Thanks.
> B.R.
> Jigong Yin
> 
> 
> 
> 发自vivo电子邮件
> 
> 

Best Regards,
Chris Lu